bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociation chemistry of hydrogen halides in water
Authors: Odde Srinivas , Mhin Byung Jin , Lee Sik , Lee Han Myoung , Kim Kwang S.
Published: May 2004

To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and Møller–Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H2O)n and X−(H3O)+(H2O)n−1 are calculated. The dissociated structures of HF and HCl can be formed for n⩾4, while those of HBr and HI can be formed for n⩾3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for n⩾4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X–H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules.
A diradical mechanism for the addition of F2 to ethene: A density functional theory study
Authors: Wang Bing-wu , Chan Leong , Chan S. P. , Chen Zhi-da , Liu Zhi-feng
Published: May 2004

The long disputed mechanism for the F2 addition to ethene is elucidated by density functional theory calculations. With hybrid functionals and a large basis set, DFT provides an excellent description for the dissociation of ground state F2, and a reasonable account for the F2⋯C2H4 van der Waals complex, which makes it possible for the modeling of reactions between elemental fluorine and organic molecules. The attack of F2 on ethene first produces a diradical intermediate, which then dissociates into two radicals CH2F–CH2⋅ and F⋅. The first step is exothermic with a low barrier around 1.8 kcal/mol, and the exothermic energy is more than enough to overcome the barrier in the second step for the homolysis of the dangling F–F bond in the diradical, although the presence of solvents and matrices environment could stabilize the diradical. Our calculations provide a coherent framework to understand this reaction not only in the gas and solution phases, but also in the matrices environment where mode-specific enhancement is observed for the addition process induced by infrared radiation.
Selective excitation in spin systems with homogeneous broadening
Authors: Fung B. M. , Ermakov Vladimir L.
Published: May 2004

We have found that the application of a weak radio frequency (rf) pulse to a spin system with indirect spin–spin (J) couplings can produce a narrow inverted peak in spectral regions where there are many overlapping peaks. Examples of three compounds with H1–1H J couplings and one with H1–13C J couplings are given. The dependences of the signals on the frequency, duration, and amplitude of the weak rf irradiation have been studied. For an rf power of γB1/2π=1.0 Hz, pulse widths longer than 0.25 s consistently produce inverted narrow peaks in spectral regions with unresolved peaks. An interpretation of the origin of this unusual result of selective excitation in spin systems with homogeneous broadening is given: the inverted signal observed can be considered as a cumulative effect of the weak rf irradiation acting on many uncorrelated transitions that are slightly off resonance on both sides of the irradiating frequency; the only role of the J couplings (or dipolar couplings in liquid crystals) is to produce a large number of closely spaced peaks that overlap with each other. Computer simulations of both coupled and noninteracting spin systems have been carried out, and the results support this interpretation.
Vibrational predissociation in the HCl dimer
Authors: Vissers G. W. M. , Oudejans L. , Miller R. E. , Groenenboom G. C. , van der Avoird A.
Published: May 2004

We present results of a combined theoretical and experimental study on the vibrational predissociation of the HCl dimer. On the theoretical side, photodissociation linewidths and product-state distributions for monomer stretch excited states with total angular momentum J=0 were computed, using the Fermi golden rule approximation. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the intermolecular stretch and geared bend modes, for both even and odd permutation symmetry. Line strengths for the transitions from the J=1, K=0 ground state to excited states with J=0 were computed using quasibound states. On the experimental side, the photofragment angular distribution method was employed to obtain complete final-state distributions for the monomer stretch excited states. Three different transitions were probed, all starting from the lower tunneling component of the ground state: the Q0R(1) transition for excitation of the acceptor stretch and the R0Q(0) transition and unresolved Q0R branch for the donor stretch excitation. We find that, in contrast to the HF dimer, the excited-state alignment of the HCl dimer, resulting from excitation using a polarized laser beam, is completely lost on the time scale of the dissociation. The agreement between theory and experiment for the product-state distributions and line strengths is reasonable. The computed lifetimes are 1–2 orders of magnitude too small, which is attributed to a deficiency in the potential energy surface.
Structure of hydrated clusters of tetrahydroisoquinoline [THIQ–(H2O)n=1,3] investigated by jet spectroscopy
Authors: Guchhait Nikhil , Banerjee Sudip , Chakraborty Avijit , Nath Debnarayan , Patwari Naresh Ganpathi , Chowdhury Mihir
Published: May 2004

The hydrated clusters of tetrahydroisoquinoline have been investigated by laser-induced fluorescence (LIF), UV–UV hole burning, and IR–UV double-resonance spectroscopy in a seeded supersonic jet. Clusters of different sizes and isomeric structures have different 0–0 transitions (origins) in the LIF spectrum. UV–UV hole burning spectroscopy has been used to identify different cluster species and their vibrational modes. The structures of the clusters have been predicted by comparing the observed OH and NH frequencies in the IR–UV double-resonance spectra with the results calculated at different levels of sophistication. It is found that the water molecules form linear and six- and eight-membered cyclic H-bonded structures at the nitrogen center of 1:1, 1:2, and 1:3 clusters, respectively.
Converged quantum calculations of HO2 bound states and resonances for J=6 and 10
Authors: Zhang Hong , Smith Sean C.
Published: May 2004

Bound and resonance states of HO2 are calculated quantum mechanically using both the Lanczos homogeneous filter diagonalization method and the real Chebyshev filter diagonalization method for nonzero total angular momentum J=6 and 10, using a parallel computing strategy. For bound states, agreement between the two methods is quite satisfactory; for resonances, while the energies are in good agreement, the widths are in general agreement. The quantum nonzero-J specific unimolecular dissociation rates for HO2 are also calculated.
Photoinduced electron transfer and geminate recombination for photoexcited acceptors in a pure donor solvent
Authors: Saik V. O. , Goun A. A. , Fayer M. D.
Published: May 2004

Photoinduced electron transfer and geminate recombination are studied for the systems rhodamine 3B (R3B+) and rhodamine 6G (R6G+), which are cations, in neat neutral N,N-dimethylaniline (DMA). Following photoexcitation of R3B+ or R6G+ (abbreviated as R+), an electron is transferred from DMA to give the neutral radical R and the cation DMA+. Because the DMA hole acceptor is the neat solvent, the forward transfer rate is very large, ∼5×1012 s−1. The forward transfer is followed by geminate recombination, which displays a long-lived component suggesting several percent of the radicals escape geminate recombination. Spectrally resolved pump–probe experiments are used in which the probe is a “white” light continuum, and the full time-dependent spectrum is recorded with a spectrometer/charge-coupled device. Observations of stimulated emission (excited state decay—forward electron transfer), the R neutral radical spectrum, and the DMA+ radical cation spectrum as well as the ground-state bleach recovery (geminate recombination) make it possible to unambiguously follow the electron transfer kinetics. Theoretical modeling shows that the long-lived component can be explained without invoking hole hopping or spin-forbidden transitions.
Glass structure and ion dynamics of lead–cadmium fluorgermanate glasses
Authors: Tambelli C. C. , Donoso J. P. , Magon C. J. , Bueno L. A. , Messaddeq Y. , Ribeiro S. J. L. , de Oliveira L. F. C. , Kosacki I.
Published: May 2004

Glass structure and fluorine motion dynamics are investigated in lead–cadmium fluorgermanate glasses by means of differential scanning calorimetry, Raman scattering, x-ray absorption (EXAFS), electrical conductivity (EC), and F19 nuclear magnetic resonance (NMR) techniques. Glasses with composition 60PbGeO3–xPbF2–yCdF2 (in mol %), with x+y=40 and x=10, 20, 30, 40, are studied. Addition of metal fluorides to the base PbGeO3 glass leads to a decrease of the glass transition temperature (Tg) and to an enhancement of the ionic conductivity properties. Raman and EXAFS data analysis suggest that metagermanate chains form the basic structural feature of these glasses. The NMR study leads to the conclusion that the F–F distances are similar to those found in pure crystalline phases. Experimental results suggest the existence of a heterogeneous glass structure at the molecular scale, which can be described by fluorine rich regions permeating the metagermanate chains. The temperature dependence of the NMR line shapes and relaxation times exhibits the qualitative and quantitative features associated with the high fluorine mobility in these systems.
A first-principles study on quasi-1D alkali metal chains within zeolite channels
Authors: Li Zhenyu , Yang Jinlong , Hou J. G. , Zhu Qingshi
Published: May 2004

We report first-principles studies on systems formed by alkali metal (Na, K, or Rb) added to zeolite ITQ-4. Geometric and electronic structures of the quasi-1D chains of intercalated alkali metal atoms at experimental loading (4 metal atoms per 32 Si) are studied. Clear differences between different kinds of alkali metal are found, with a general trend of decreased ionization and less metallic character for the lighter alkali metals. Within the zeolite channels, it is possible to form insulated and metallic alkali metal chains by doping Na or Rb. Agreeing with experiments, only Rb here is found to be a good candidate to generate inorganic electride. We also predict that a large quantity of Na can be doped into the zeolite channel, while no more than 4 Rb per 16 Si can be doped.
Nuclear magnetic resonance study of the vapor contribution to diffusion in silica glasses with micrometer pores partially filled with liquid cyclohexane or water
Authors: Ardelean Ioan , Farrher German , Mattea Carlos , Kimmich Rainer
Published: May 2004

The contribution of the vapor phase to molecular diffusion in porous silica glass (Vitrapor#5; mean pore diameter 1 micrometer) partially filled with cyclohexane (nonpolar) or water (polar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane increased up to ten times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. A two-phase exchange theory is presented accounting well for all experimental diffusion features. The diffusion behavior in the samples with micrometer pores under investigation here is in contrast to previous findings for the same solvents in a material with nanometer pores (Vycor; mean pore diameter 4 nm) where the fast-exchange limit had to be assumed [Ardelean et al., J. Chem. Phys. 119, 10358 (2003)]. It is concluded that the pore size plays a crucial role for the relevance of molecular exchange limits relative to the experimental diffusion/exchange time.
Density-functional study of the cycloaddition of acrylonitrile on the Si(100) surface
Authors: Cobian M. , Ilakovac V. , Carniato S. , Capron N. , Boureau G. , Hirschl R. , Hafner J.
Published: May 2004

Using a density functional approach, we have explored the cycloaddition of acrylonitrile on the Si(100) surface. The buckling of the surface dimers characteristic for the (2×1) reconstructed surface is shown to favor structures with a dipolar moment such as the resonant form of acrylonitrile with cumulative double bonds. The bond of acrylonitrile via a single C atom is a possible intermediate leading to the nitrile structure of the adsorbed molecule.
The characteristic curve and site-selective laser excitation of local relaxation in glass
Authors: Canning J.
Published: May 2004

The so-called characteristic curve describing photosensitivity change is elaborated and shown to be a powerful tool for understanding and characterizing photosensitive growth both at a fundamental and practical level. It has been used successfully to diagnose when optimal hypersensitization has been achieved and the physical basis for this is explained. By way of example, previous results using 355 nm hypersensitization are re-examined. Evidence of single site-selective glass relaxation through direct laser excitation offers a new approach to accessing and studying induced relaxations.
Thickness-dependent ordering of water layers at the NaCl(100) surface
Authors: Arsic J. , Kaminski D. M. , Radenovic N. , Poodt P. , Graswinckel W. S. , Cuppen H. M. , Vlieg E.
Published: May 2004

We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid–liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid–liquid interface, the ordering appears to be correlated with the solubility.
On the premelting features in sodium clusters
Authors: Calvo F. , Spiegelman F.
Published: May 2004

Melting in Nan clusters described with an empirical embedded-atom potential has been reexamined in the size range 55⩽n⩽147 with a special attention at sizes close to 130. Contrary to previous findings, premelting effects are also present at such medium sizes, and they turn out to be even stronger than the melting process itself for Na133 or Na135. These results indicate that the empirical potential is qualitatively inadequate to model sodium clusters.
Crossed beam studies of the reactions of atomic oxygen in the ground P3 and first electronically excited D1 states with hydrogen sulfide
Authors: Balucani Nadia , Stranges Domenico , Casavecchia Piergiorgio , Volpi Gian Gualberto
Published: May 2004

The reactions of both ground, P3, and electronically excited, D1, oxygen atoms with hydrogen sulfide, H2S, have been investigated by means of the crossed molecular beams method with mass spectrometric detection at different collision energies. Amongst the possible reaction channels those leading to HSO+H for the O(3P) reaction and to HSO/HOS+H and SO+H2 for the O(1D) reaction have been identified and investigated. The dynamics of the channels leading to HSO/HOS+H are elucidated for the reactions of both states and the trend with increasing the collision energy analyzed. Noteworthily, the formation of SO+H2 products appears to be an open channel for the O(1D) reaction, at least for the highest collision energy investigated (11.8 kcal/mol). Finally, the recent experimental and theoretical estimates of the enthalpy of formation of the HSO radical have been critically analyzed to evaluate their conformity with the present experimental data.
Thermodynamic properties and aggregate formation of surfactant-like molecules from theory and simulation
Authors: Herdes Carmelo , Pàmies Josep C. , Marcos Rosa M. , Vega Lourdes F.
Published: May 2004

The goal of this work is twofold: to predict the phase equilibria behavior of simplified surfactant models and to predict the population of aggregates as a function of pressure. We compare Monte Carlo simulation results of these systems with predictions from a modified version of the statistical associating fluid theory (soft-SAFT). Surfactant-like molecules are modeled as Lennard-Jones chains of tangent segments with one or two association sites. We study the influence of the number and location of the association sites on the thermodynamic properties and fraction of nonbonded molecules in all cases. The influence of the chain length is also investigated for a particular location of the sites. Results are compared with NPT Monte Carlo simulations to test the accuracy of the theory, and to study the molecular configurations of the system. Soft-SAFT is able to quantitatively predict the MC PVT results, independently of the location of the association sites. The theory is also able to capture the qualitative trend of the population of aggregates with pressure. Quantitative agreement is only obtained for specific locations of the sites.
Calculations of helical twisting powers from intermolecular torques
Authors: Earl David J. , Wilson Mark R.
Published: May 2004

We present a Monte Carlo molecular simulation method that calculates the helical twisting power of a chiral molecule by sampling intermolecular torques. The approach is applied to an achiral nematic liquid crystalline system, composed of Gay–Berne particles, that is doped with chiral molecules. Calculations are presented for six chiral dopant molecules and the results show a good correlation with the sign and magnitude of experimentally determined helical twisting powers.
Fluorescence quenching by reversible excimer formation: Kinetics and yield predictions for a classical potential association–dissociation model
Authors: Naumann Wolfgang
Published: May 2004

Fluorescence quenching by reversible excimer formation is studied on the assumption that excimer formation and dissociation can be modelled as entering and leaving the attractive region of an monomer excited-monomer interaction potential by diffusion. To get some general insight in the kinetic consequences of such a type of modelling, the simple case of an attractive square-well potential is investigated. It is shown that three different kinetic regimes have to be distinguished: Two “reversible” ones in case of slow excimer radiative decay, in which the quenching kinetics can be formulated by Markovian or non-Markovian rate equations with both excimer formation and excimer dissociation terms, and an effectively “irreversible” regime if the excimer radiative decay is too rapid to allow the excimer equilibration. In the latter case a dissociation coefficient can no longer be defined and the quenching kinetics can only be predicted on the basis of generalized rate equations of a net-excimer-formation type. It is shown how the quenching constant formula must be generalized to be applicable in all kinetic situations.
Relaxation of the electrical double layer after an electron transfer approached by Brownian dynamics simulation
Authors: Grün F. , Jardat M. , Turq P. , Amatore C.
Published: May 2004

In this paper, the dynamical properties of the electrochemical double layer following an electron transfer are investigated by using Brownian dynamics simulations. This work is motivated by recent developments in ultrafast cyclic voltammetry which allow nanosecond time scales to be reached. A simple model of an electrochemical cell is developed by considering a 1:1 supporting electrolyte between two parallel walls carrying opposite surface charges, representing the electrodes; the solution also contains two neutral solutes representing the electroactive species. Equilibrium Brownian dynamics simulations of this system are performed. To mimic electron transfer processes at the electrode, the charge of the electroactive species are suddenly changed, and the subsequent relaxation of the surrounding ionic atmosphere are followed, using nonequilibrium Brownian dynamics. The electrostatic potential created in the center of the electroactive species by other ions is found to have an exponential decay which allows the evaluation of a characteristic relaxation time. The influence of the surface charge and of the electrolyte concentration on this time is discussed, for several conditions that mirror the ones of classical electrochemical experiments. The computed relaxation time of the double layer in aqueous solutions is found in the range 0.1 to 0.4 ns for electrolyte concentrations between 0.1 and 1 mol L−1 and surface charges between 0.032 and 0.128 C m−2.
Single-photon vacuum-ultraviolet laser-pulsed-field ionization-photoelectron studies of trans- and cis-1-bromopropenes
Authors: Woo H. K. , Wang P. , Lau K.-C. , Xing X. , Ng C. Y.
Published: May 2004

The vacuum-ultraviolet (VUV) pulsed-field ionization-photoelectron (VUV-PFI-PE) spectra of trans-1-bromopropene (trans-CH3CH=CHBr) and cis-1-bromopropene (cis-CH3CH=CHBr) have been measured in the energy region of 74 720–76 840 cm−1. The simulation of fine structures observed in the origin VUV-PFI-PE vibrational bands of these molecules has provided the ionization energies (IEs) of trans-1-bromopropene and cis-1-bromopropene to be 74 779.3±2.0 cm−1 (9.2715±0.0002 eV) and 75 140.2±2.0 cm−1 (9.3162±0.0002 eV), respectively. The vibrational bands resolved in these VUV-PFI-PE spectra at energies 0–1700 cm−1 above the IEs of trans-1-bromopropene and cis-1-bromopropene have been assigned based on theoretical vibrational frequencies and calculated Franck–Condon factors for the ionization transitions.
Fabrication of crystalline organic waveguides with an exceptionally large electro-optic coefficient
Authors: Geis W. , Sinta R. , Mowers W. , Deneault S. J. , Marchant M. F. , Krohn K. E. , Spector S. J. , Calawa D. R. , Lyszczarz T. M.
Published: May 2004

Single-crystal optical waveguides of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), an organic material with a large electro-optic coefficient, have been obtained. DAST decomposes at its melting temperature, making its growth from the melt difficult. However, graphoepitaxy allows for >1 mm s−1 growth, 1×105 times faster than conventional techniques, and produces crystals of the correct dimensions for optical waveguides, 1–15 μm on a side and 5–10 mm long. The crystals grow with the c-axis normal to the substrate, and with in-plane orientation determined by lithographic patterning. The electro-optic coefficient dn/dE is 600±300 pm V−1 at 1.55 μm wavelength. Optical losses are <10 dB cm−1.
Combined Monte Carlo and molecular dynamics simulation of hydrated 18:0 sphingomyelin–cholesterol lipid bilayers
Authors: Khelashvili George A. , Scott H. L.
Published: May 2004

We have carried out atomic level molecular dynamics and Monte Carlo simulations of hydrated 18:0 sphingomyelin (SM)–cholesterol (CHOL) bilayers at temperatures of 20 and 50 °C. The simulated systems each contained 266 SM, 122 CHOL, and 11861 water molecules. Each simulation was run for 10 ns under semi-isotropic pressure boundary conditions. The particle-mesh Ewald method was used for long-range electrostatic interactions. Properties of the systems were calculated over the final 3 ns. We compare the properties of 20 and 50 °C bilayer systems with each other, with experimental data, and with experimental and simulated properties of pure SM bilayers and dipalmitoyl phospatidyl choline (DPPC)–CHOL bilayers. The simulations reveal an overall similarity of both systems, despite the 30 °C temperature difference which brackets the pure SM main phase transition. The area per molecule, lipid chain order parameter profiles, atom distributions, and electron density profiles are all very similar for the two simulated systems. Consistent with simulations from our lab and others, we find strong intramolecular hydrogen bonding in SM molecules between the phosphate ester oxygen and the hydroxyl hydrogen atoms. We also find that cholesterol hydroxyl groups tend to form hydrogen bonds primarily with SM carbonyl, methyl, and amide moieties and to a lesser extent methyl and hydroxyl oxygens.
Concentration fluctuation effects on the phase behavior of compressible diblock copolymers
Authors: Cho Junhan
Published: May 2004

A Hartree analysis has been performed for compressible diblock copolymers of incompatible pairs to investigate the concentration fluctuation effects on their microphase separation behavior. The free energy in the Hartree analysis is obtained from the self-consistent correction to its mean-field cousin, which was recently formulated for such copolymer systems. The mean-field phase diagram is shown to be significantly affected by the fluctuation effects as the copolymer chain size N is lowered. An effective interaction χcRPA, which carries not only the change in contact interactions but also the compressibility difference between block components, plays a key role in understanding of the phase behavior and the pressure responses of various thermodynamic transitions for the copolymers with finite sizes. In particular, a symmetric copolymer at disorder–to–lamella transition is found to satisfy NχcRPA(q*)=10.495+41.022N−1/3 when evaluated at a characteristic wave number q* for ordered microphases.
Type-II interband transition of ZnS0.78Te0.22/ZnTe single quantum wells
Authors: Bahng Jae Ho , Moon S. J. , Lee K. H. , Choi J. C. , Jeong K. , Park H. L.
Published: May 2004

Optical properties of ZnS0.78Te0.22/ZnTe single quantum wells grown on GaAs (100) substrates by hot wall epitaxy technique with varying the ZnS0.78Te0.22 well width from 0.3 to 1.8 nm were investigated by photoluminescence (PL) measurements at low temperature and by temperature-dependent PL measurements. PL results show the evidence of type-II transition and their peak energy shifts to higher energies as the ZnS0.78Te0.22 well width decreases. In addition, temperature-dependent PL measurements show the increase of the activation energy as the well thickness decreases, indicating the increase of confinement effect. This study makes it possible to introduce proper band diagram for this structure, and can give very useful information on their device applications.
Effect of surface NH3 anneal on the physical and electrical properties of HfO2 films on Ge substrate
Authors: Wu Nan , Zhang Qingchun , Zhu Chunxiang , Yeo Chia Chin , Whang S. J. , Chan D. S. H. , Li M. F. , Cho Byung Jin , Chin Albert , Kwong Dim-Lee , Du A. Y. , Tung C. H. , Balasubramanian N.
Published: May 2004

Metal-oxide-semiconductor capacitors were fabricated on germanium substrates by using metalorganic-chemical-vapor-deposited HfO2 as the dielectric and TaN as the metal gate electrode. It is demonstrated that a surface annealing step in NH3 ambient before the HfO2 deposition could result in significant improvement in both gate leakage current and the equivalent oxide thickness (EOT). It was possible to achieve a capacitor with an EOT of 10.5 Å and a leakage current of 5.02×10−5 A/cm2 at 1 V gate bias. X-ray photoelectron spectroscopy analysis indicates the formation of GeON during surface NH3 anneal. The presence of Ge was also detected within the HfO2 films. This may be due to Ge diffusion at the high temperature (∼400 °C) used in the chemical-vapor deposition process.
Comparison of predicted ferromagnetic tendencies of Mn substituting the Ga site in III–V’s and in I–III–VI2 chalcopyrite semiconductors
Authors: Zhao Yu-Jun , Mahadevan Priya , Zunger Alex
Published: May 2004

We report density-functional calculations of the ferromagnetic (FM) stabilization energy δ=EFM−EAFM for differently oriented Mn pairs in III–V’s (GaN, GaP, GaAs) and chalcopyrite (CuGaS2, CuGaSe2, CuGaTe2) semiconductors. Ferromagnetism is found to be the universal ground state (δ<0) in all cases. The order of FM stability in III–V’s is GaN>GaP>GaAs, whereas in chalcopyrites it is CuGaS2>CuGaSe2>CuGaTe2. Considering both groups, the order is GaN→GaP→GaAs→CuGaS2→CuGaSe2→GaSb≈CuGaTe2. The stronger FM stabilization in III–V’s is attributed to the stronger covalent coupling between the Mn 3d and the anion p orbitals. In contrast to expectations based on Ruderman–Kittel–(Kasuya)–Yosida, (i) all Mn–Mn pair separations show FM, with no FM to antiferromagnetic oscillations and, (ii) FM is orientationally dependent, with 〈110〉 Mn–Mn pairs being the most FM.
Imaging phonons in a fcc Pu–Ga alloy by thermal diffuse x-ray scattering
Authors: Wong Joe , Wall M. , Schwartz A. J. , Xu R. , Holt M. , Hong Hawoong , Zschack P. , Chiang T.-C.
Published: May 2004

X-ray thermal diffuse scattering intensity patterns from phonons in a fcc δ-Pu–Ga alloy have been recorded using an 18 keV undulator x-ray beam with a beam diameter of 25 μm. The results are consistent with patterns calculated using the Born–von Karman force constant model of lattice dynamics, and support the pronounced softening of the transverse acoustic branch along the [111] direction observed from inelastic x-ray scattering measurements. This work demonstrates the feasibility of using a “large-grain, small beam” approach to study lattice properties, such as phonon dispersion curves, of materials not readily available in the form of large single crystals.
Toward improved and tunable polymer field-effect transistors
Authors: Edman Ludvig , Swensen James , Moses Daniel , Heeger Alan J.
Published: May 2004

We report an electrochemical method to improve charge injection in thin-film field-effect transistors fabricated with semiconducting polymers. By having ions, which are mobile only at elevated temperatures, in the active material [a mixture of a soluble poly(para-phenylene vinylene) copolymer, a crown ether and a LiCF3SO3 salt] we create electric double layers at the drain/source Au electrode interfaces by applying a low voltage (V=2 V) at T=85 °C for a short time (t∼1–5 min). After cooling to room temperature under applied voltage, we demonstrate significantly improved transistor performance. In addition, we present evidence of reversible electrochemical doping in this active material.
Chaperoned alchemical free energy simulations: A general method for QM, MM, and QM/MM potentials
Authors: Yang Wei , Bitetti-Putzer Ryan , Karplus Martin
Published: May 2004

A general method for alchemical free energy simulations using QM, MM, and QM/MM potential is developed by introducing “chaperones” to restrain the structures, particularly near the end points. A calculation of the free energy difference between two triazole tautomers in aqueous solution is used to illustrate the method.
Time reversal kaleidoscope: A smart transducer for three-dimensional ultrasonic imaging
Authors: Montaldo Gabriel , Palacio Delphine , Tanter Mickael , Fink Mathias
Published: May 2004

The design of two dimensional (2D) arrays for three dimensional (3D) ultrasonic imaging is a major challenge in medical and nondestructive applications. Thousands of transducers are typically needed for beam focusing and steering in 3D volumes. Here, we report a promising approach for producing 3D images with a small number of transducers using the combined concepts of time reversal mirrors and chaotic reverberating cavities. Due to multiple reverberations inside the cavity, a “kaleidoscopic” transducer array is created with thousands of virtual transducers equivalent to 2D matrices. Beyond the scope of 3D medical imaging, this work leads to the concept of “smart” transducer.
Preparation and phase transformation of highly ordered TiO2 nanodot arrays on sapphire substrates
Authors: Chen Po-Lin , Kuo Cheng-Tzu , Pan Fu-Ming , Tsai Tzeng-Guang
Published: May 2004

Ordered nanodot arrays of titanium oxide (TiO2) were prepared from an epitaxial Al/TiN bilayered film on a sapphire substrate by electrochemical anodization of the TiN layer using a nanoporous anodic aluminum oxide (AAO) film as the template. The nanodots with an average diameter of about 60 nm can faithfully duplicate the size, shape, and hexagonal pore pattern of the AAO nanopores. The phase development of the isolated TiO2 nanodots is very much different from TiO2 thin films and powders. After high temperature annealing, the nanodots are polycrystalline and consist of a mixed phase of anatase and rutile instead of single rutile phase. We expect that TiO2 nanodots with a single phase of anatase can be realized as long as the size of the nanodots is smaller than the critical nuclei size for rutile formation.
H-related defect complexes in HfO2: A model for positive fixed charge defects
Authors: Kang Joongoo , Lee E.-C. , Chang K. J. , Jin Young-Gu
Published: May 2004

Based on first-principles theoretical calculations, we investigate the hydrogenation effect on the defect properties of oxygen vacancies (VO) in HfO2. A defect complex of VO and H behaves as a shallow donor for a wide range of Fermi levels, with a positive charge state, and this complex is energetically stable against its dissociation into VO and H. We suggest that the VO–H complex is responsible for the formation of positive fixed charges, which neutralize negative fixed charges during the postannealing process of SiOx/HfO2 stack.
Influence of buffer layers on the performance of polymer solar cells
Authors: Zhang F. L. , Gadisa A. , Inganäs O. , Svensson M. , Andersson M. R.
Published: May 2004

The influence of different anode buffer layers on the performance of solar cells based on blends of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (DiO-PFDTBT), acting as electron donor, and [6,6]-phenyl-C61-butyric acid methylester (PCBM), acting as electron acceptor was studied. The buffer layers were constructed from different forms of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT-PSS). Variations in open-circuit voltage, short-circuit current, and fill factor were observed. These were attributed to the slight rearrangement of the insulating layer PSS on top of PEDOT, which altered the charge injection at the interface between anode and photoactive layer.
Control of the spin polarization of photoelectrons/photoions using short laser pulses
Authors: Nakajima Takashi
Published: May 2004

We present a generic pump-probe scheme to control spin polarization of photoelectrons/photoions by short laser pulses. By coherently exciting fine structure manifolds of a multi-valence-electron system by the pump laser, a superposition of fine structure states is created. Since each fine structure state can be further decomposed into a superposition of various spin states of valence electrons, each spin component evolves differently in time. This means that varying the time delay between the pump and probe lasers leads to the control of spin states. Specific theoretical results are presented for two-valence-electron atoms, in particular for Mg, which demonstrate that not only the degree of spin polarization but also its sign can be manipulated through time delay. Since the underline physics is rather general and transparent, the presented idea may be potentially applied to nanostructures such as quantum wells and quantum dots.
High mobility of pentacene field-effect transistors with polyimide gate dielectric layers
Authors: Kato Yusaku , Iba Shingo , Teramoto Ryohei , Sekitani Tsuyoshi , Someya Takao , Kawaguchi Hiroshi , Sakurai Takayasu
Published: May 2004

Polyimide gate dielectric layers cured at 180 °C have been employed to fabricate high-quality pentacene field-effect transistors on polyethylenenaphthalate-based films. The surface roughness (root-mean square) of gate dielectric layers characterized by atomic force microscopy is only 0.2 nm, while that of the base film is 1 nm. The transistors with pentacene channel layers deposited on 990 nm polyimide gate dielectric layers attain the on/off ratio of 106 and mobility of 0.3 cm2/V s. Furthermore, by decreasing the thickness of polyimide gate dielectric layers down to 540 nm, the mobility is enhanced up to 1 cm2/V s.
Amorphization of silicon carbide by carbon displacement
Authors: Devanathan R. , Gao F. , Weber W. J.
Published: May 2004

We have used molecular dynamics simulations to examine the possibility of amorphizing silicon carbide (SiC) by exclusively displacing C atoms. At a defect generation corresponding to 0.2 displacements per atom, the enthalpy surpasses the level of melt-quenched SiC, the density decreases by about 15%, and the radial distribution function shows a lack of long-range order. Prior to amorphization, the surviving defects are mainly C Frenkel pairs (67%), but Si Frenkel pairs (18%) and antisite defects (15%) are also present. The results indicate that SiC can be amorphized by C sublattice displacements. Chemical short-range disorder, arising mainly from Frenkel pair production, plays a significant role in the amorphization.
Microwave radiation force and torque on a disk resonator excited by a circularly polarized plane wave
Authors: Makarov S. , Kulkarni S.
Published: May 2004

A numerical simulation method [S. Makarov and S. Kulkarni, Appl. Phys. Lett. 84, 1600 (2004)] is used in order to determine the radiation force and radiation torque on a parallel-plate disk resonator, whose size is comparable to wavelength. The method is based on the MOM solution of the electric-field integral equation, accurate calculation of the near field, and removal of the self-interaction terms responsible for the pinch effect. The local force/torque distribution at the normal incidence of a circularly polarized plane wave is found. It is observed that, at the resonance, the individual disks are subject to unexpectedly large local force densities, despite the fact that the net radiation force on the resonator remains very small. On the other hand, the total axial torque on the disk resonator also increases at the resonance.
Experimental investigation of coherent structures in a low-energy electron beam
Authors: Bettega G. , Cavaliere F. , Illiberi A. , Pozzoli R. , Romé M. , Cavenago M. , Tsidulko Yu.
Published: May 2004

A sharp transition to a space-charge dominated regime is induced in a low-energy electron beam produced in a Malmberg–Penning trap by increasing the emission current of the source. The transition is characterized by the appearance of a region, around the axis of the beam, not accessible to beam electrons, and by the fast development of coherent structures in the remaining electron plasma, due to the sharp increase of local vorticity. The results are interpreted in the framework of a cold fluid drift–Poisson model, and using a three-dimensional particle-in-cell simulation code.
Ordered domains and boundary structure in Ba(Cd1/3Ta2/3)O3 perovskite dielectrics
Authors: Sun J. , Liu Shaojun , Newman N. , Smith David J.
Published: May 2004

Ordered domains and boundaries have been observed in Ba(Cd1/3Ta2/3)O3 perovskite dielectrics by electron microscopy. Individual nanoscale domains were found to be well ordered with hexagonal symmetry. Electron diffraction revealed a twinned crystallographic relationship between domains. High-resolution electron microscopy showed further details of the boundary structure. Conservative twin boundaries along the (001) and (110) planes and nonconservative antiphase boundaries with a projected displacement vector of the type [001] were observed. The relationship between the energy and structure of the domain boundaries is briefly discussed.
Field-effect tuning of carrier density in Nd1.2Ba1.8Cu3Oy thin films
Authors: Cassinese A. , De Luca G. M. , Prigiobbo A. , Salluzzo M. , Vaglio R.
Published: May 2004

Using a field effect device we modified the number of holes in the surface layers of 4 to 10 unit cell Nd1.2Ba1.8Cu3Oy (NBCO) epitaxial films grown on (100) SrTiO3 substrates. The results obtained on a set of 12 devices demonstrate that it is possible to induce reversible changes of the hole density of NBCO films by field effect. It is found that the field effect becomes less pronounced increasing the film thickness. Insulating–superconducting transition was observed in one 8 unit cell NBCO field effect device.
Emission from the higher-order excitons in ZnO films grown by laser molecular-beam epitaxy
Authors: Tsukazaki A. , Ohtomo A. , Kawasaki M. , Makino T. , Chia C. H. , Segawa Y. , Koinuma H.
Published: May 2004

Epitaxial ZnO thin films were grown by laser molecular-beam epitaxy on lattice-matched ScAlMgO4 substrates following the deposition and annealing of suitable buffer layers. The samples were characterized by low-temperature photoluminescence (PL), absorption, and reflectivity measurements. PL from higher order (n=2) excitons (A exciton) was observed at temperatures lower than 40 K. The absorption spectrum contained lines and the reflection spectrum exhibited anomalies that were assigned to the excited-states (n=2,3) of A and B excitons. The optical quality could be improved dramatically by using annealed ZnO or MgZnO buffer layers.
Optimal control of ultrafast selection
Authors: Yokoyama K. , Teranishi Y. , Toya Y. , Shirai T. , Fukuda Y. , Aoyama M. , Akahane Y. , Inoue N. , Ueda H. , Yamakawa K. , Yokoyama A. , Yamada H. , Yabushita A. , Sugita A.
Published: May 2004

Optimal laser control for ultrafast selection of closely lying excited states whose energy separation is smaller than the laser bandwidth is reported on the two-photon transition of atomic cesium; Cs(6S→7DJ, J=5/2 and 3/2). Selective excitation was carried out by pulse shaping of ultrashort laser pulses which were adaptively modulated in a closed-loop learning system handling eight parameters representing the electric field. Two-color fluorescence from the respective excited states was monitored to measure the selectivity. The fitness used in the learning algorithm was evaluated from the ratio of the fluorescence yields. After fifty generations, a pair of nearly transform-limitted pulses were obtained as an optimal pulse shape, proving the effectiveness of the “Ramsey fringes” mechanism. The contrast of the selection ratio was improved by ∼30% from the simple “Ramsey fringes” experiment.
The systematics of freshwater crayfish of the genus Cherax Erichson (Decapoda : Parastacidae) in eastern Australia re-examined using nucleotide sequences from 12S rRNA and 16S rRNA genes
Authors: Munasinghe D. H. N. , Burridge C. P. , Austin C. M.
Published: May 2004

Nucleotide sequence data were used to re-examine systematic relationships and species boundaries within the genus Cherax from eastern Australia. Partial sequences were amplified from the 12S (~365 bp) and 16S (~545 bp) rRNA mitochondrial gene regions. Levels of intra- and inter-specific divergence for Cherax species were very similar between the two gene regions and similar to that reported for other freshwater crayfish for 16S rRNA. Phylogenetic analyses using the combined data provided strong support for a monophyletic group containing 11�eastern Australian species and comprising three well-defined species-groups: the 'C. destructor' group containing three species, the 'C. cairnsensis' group containing four species and the 'C. cuspidatus' group containing two species. Cherax dispar and C. robustus are distinct from all other species and each other. In addition, two northern Australian and a New Guinean species were placed in the 'Astaconephrops' group, which is the sister-group to the eastern Australian Cherax lineage. Several relationships were clarified, including: the status of northern and southern C. cuspidatus as separate species; a close relationship between C. cairnsensis and C. depressus; the validity of C. rotundus and C. setosus as separate species and their close affinities with C. destructor; and the distinctiveness of the northern forms of Cherax. The analysis of the 12S rRNA and 16S rRNA data is highly concordant with the results of previous allozyme studies.
Automated structure prediction of weakly homologous proteins on a genomic scale
Authors: Zhang Yang , Skolnick Jeffrey
Published: May 2004

We have developed
tasser
, a hierarchical approach to protein structure prediction that consists of template identification by threading, followed by tertiary structure assembly via the rearrangement of continuous template fragments guided by an optimized C
α
and side-chain-based potential driven by threading-based, predicted tertiary restraints.
tasser
was applied to a comprehensive benchmark set of 1,489 medium-sized proteins in the Protein Data Bank. With homologues excluded, in 927 cases, the templates identified by our threading algorithm
prospector_3
have a rms deviation from native <6.5 Å with ≈80% alignment coverage. After template reassembly, this number increases to 1,172. This shows significant and systematic improvement of the final models with respect to the initial template alignments. Furthermore, significant improvements in loop modeling are demonstrated. We then apply
tasser
to the 1,360 medium-sized ORFs in the
Escherichia coli
genome; ≈920 can be predicted with high accuracy based on confidence criteria established in the Protein Data Bank benchmark. These results from our unprecedented comprehensive folding benchmark on all protein categories provide a reliable basis for the application of
tasser
to structural genomics, especially to proteins of low sequence identity to solved protein structures.
Lipids in the inner membrane of dormant spores of
Bacillus
species are largely immobile
Authors: Cowan Ann E. , Olivastro Elizabeth M. , Koppel Dennis E. , Loshon Charles A. , Setlow Barbara , Setlow Peter
Published: May 2004

Bacterial spores of various
Bacillus
species are impermeable or exhibit low permeability to many compounds that readily penetrate germinated spores, including methylamine. We now show that a lipid probe in the inner membrane of dormant spores of
Bacillus megaterium
and
Bacillus subtilis
is largely immobile, as measured by fluorescence redistribution after photobleaching, but becomes free to diffuse laterally upon spore germination. The lipid immobility in and the slow permeation of methylamine through the inner membrane of dormant spores may be due to a significant (1.3- to 1.6-fold) apparent reduction of the membrane surface area in the dormant spore relative to that in the germinated spore, but is not due to the dormant spore's high levels of dipicolinic acid and divalent cations.
Similar gene expression patterns characterize aging and oxidative stress in
Drosophila melanogaster
Authors: Landis Gary N. , Abdueva Diana , Skvortsov Dmitriy , Yang Junsheng , Rabin Beth E. , Carrick James , Tavaré Simon , Tower John
Published: May 2004

Affymetrix GeneChips were used to measure RNA abundance for ≈13,500
Drosophila
genes in young, old, and 100% oxygen-stressed flies. Data were analyzed by using a recently developed background correction algorithm and a robust multichip model-based statistical analysis that dramatically increased the ability to identify changes in gene expression. Aging and oxidative stress responses shared the up-regulation of purine biosynthesis, heat shock protein, antioxidant, and innate immune response genes. Results were confirmed by using Northerns and transgenic reporters. Immune response gene promoters linked to GFP allowed longitudinal assay of gene expression during aging in individual flies. Immune reporter expression in young flies was partially predictive of remaining life span, suggesting their potential as biomonitors of aging.