bluuebunny/crossref_metadata_2025 huggingface dataset:
High-temperature photomodulated thermoreflectance measurements on phosphorus implanted and annealed silicon wafers
Authors: Othonos Andreas , Christofides Constantinos , Loizidou Efi
Published: November 2003

Photomodulated thermoreflectance measurements between 300 and 650 K on phosphorus implanted and annealed silicon wafers are reported. The change of the photothermal amplitude and phase as a function of temperature is discussed. Several measurements have been performed on silicon wafers annealed at various temperatures in the range of 300 to 1100 °C. The activation energy of the local annealing process was also estimated to be 0.17 eV.
Ex-situ investigation of indium segregation in InGaAs/GaAs quantum wells using high-resolution x-ray diffraction
Authors: Martini S. , Quivy A. A. , da Silva M. J. , Lamas T. E. , da Silva E. C. F. , Leite J. R. , Abramof E.
Published: November 2003

Calculations using the dynamical theory of diffraction together with a sample model which considers the segregation of indium atoms were employed to fit the high-resolution x-ray spectra of strained InGaAs/GaAs quantum wells grown by molecular-beam epitaxy. The segregation coefficients obtained from the best fits to the experimental data of samples grown at different temperatures are in excellent agreement with the expected values and confirm that x-ray diffraction is a valuable tool for the investigation of the segregation phenomenon.
Delayed formation dynamics of HArF and HKrF in rare-gas matrices
Authors: Bihary Z. , Chaban G. M. , Gerber R. B.
Published: November 2003

HArF and HKrF are chemically bound rare-gas compounds that have been produced by photolysis of HF and subsequent thermal annealing in the respective rare-gas matrices. In this paper we present a computational study of the delayed, thermally induced formation of these molecules in the matrix. Using realistic potentials for the molecular and guest–host interactions, the potential energy along the minimum energy paths for formation is evaluated, and thermal transition rates are computed using a Monte Carlo transition state method. A closely packed, dissociated configuration of the molecular fragments is found to play an important role, both as the possible trapping site of the photolyzed fragments, and as an intermediate structure for diffusion-controlled formation. The computed threshold temperatures of formation for the HArF and HKrF molecules at different matrix sites are in good agreement with experimental findings and with previous site assignments for these molecules.
Accurate potential energy surfaces for the study of lithium–hydrogen ionic reactions
Authors: Martinazzo R. , Tantardini G. F. , Bodo E. , Gianturco F. A.
Published: November 2003

Three-dimensional potential energy surfaces (PESs) have been computed, and numerically fitted, for the two lowest electronic states of the LiH2+ system, which are of importance for the astrophysically relevant LiH++H→Li++H2 and LiH+H+→Li+H2+ exoergic reactions. We extend the recently computed 11 000 multi reference valence bond ab initio energy values [Martinazzo et al., Chem. Phys. 287, 335 (2003)] with 600 multireference configuration interaction calculations with complete active self-consistent field reference functions and a large Li(12s10p4d1f)/H(8s6p3d1f) basis set. We have fitted the full set of energy values with a modified Aguado–Paniagua ansatz that correctly takes into account in this ionic system the important long-range contributions to the potential. Calibration calculations on the three-body potential term and the use of essentially exact results for the two-body contributions allow us to estimate the overall accuracy of the analytic PESs to be within that required for accurate quantum scattering calculations. The above reactions can be treated adiabatically because of the large energy gap separating the two electronic states. The relevant potential energy surfaces have a very different shape. On the one hand, the ground-state PES shows a simple structure, with a downhill route to the products and a shallow well at the C2v geometry which lies 0.286 eV below the Li++H2 asymptote. On the other hand, the first excited state is characterized by one deep, dipole-charge well which lies 1.315 eV below the LiH+H+ asymptote, one charge-induced dipole well 0.586 eV below the Li+H2+ asymptote, and a saddle point between them which lies 0.227 eV below the LiH+H+ asymptote. A conical intersection with the second excited state has been found but not yet studied in detail, since we deemed it to be of no direct relevance for the above reactions.
The effect of surface roughness on the adhesion of elastic plates with application to biological systems
Authors: Persson B. N. J. , Gorb S.
Published: November 2003

We study the influence of surface roughness on the adhesion of elastic plates. Most real surfaces have roughness on many different length scales, and this fact is taken into account in our analysis. We consider in detail the case when the surface roughness can be described as a self-affine fractal, and study the plate–substrate pull-off force as a function of the surface roughness. Based on the theoretical results we discuss adhesion in biological systems, focusing on the adhesive pads of lizards.
Solid state quantum computer development in silicon with single ion implantation
Authors: Schenkel T. , Persaud A. , Park S. J. , Nilsson J. , Bokor J. , Liddle J. A. , Keller R. , Schneider D. H. , Cheng D. W. , Humphries D. E.
Published: November 2003

Spawned by the finding of efficient quantum algorithms, the development of a scalable quantum computer has emerged as a premiere challenge for nanoscience and nanotechnology in the last years. Spins of electrons and nuclei in P31 atoms embedded in silicon are promising quantum bit (qubit) candidates. In this article we describe single atom doping strategies and the status of our development of single atom qubit arrays integrated with control gates and readout structures in a “top down” approach. We discuss requirements for P31 qubit array formation by single ion implantation, and integration with semiconductor processing.
Large scale molecular dynamics simulation of nucleation in supercooled NaCl
Authors: Koishi Takahiro , Yasuoka Kenji , Ebisuzaki Toshikazu
Published: November 2003

Nucleation of NaCl was reproduced by using molecular dynamics simulation. We carried out MD simulations in the 13 824 ion system under the free boundary condition. Critical nucleus size, nucleation time lag, and nucleation rate were directly estimated from simulation results without using nucleation theory. We also carried out MD simulations in the 13 824 and 125 000 ion systems under the periodic boundary condition to compare the results with those of the free boundary condition. A single crystal and a polycrystal were formed in our simulation. We investigated the difference of the nucleation process of a single crystal and a polycrystal. All simulations were calculated by using the special purpose computer, MDGRAPE-2, for molecular dynamics simulation.
A Kirkwood–Buff derived force field for sodium chloride in water
Authors: Weerasinghe Samantha , Smith Paul E.
Published: November 2003

A force field for the simulation of mixtures of sodium chloride and water is described. The model is specifically designed to reproduce the experimentally determined Kirkwood–Buff integrals as a function of salt concentration, ensuring that a good representation of the solution activity is obtained. In addition, the model reproduces many of the known properties of sodium chloride solutions including the density, isothermal compressibility, ion diffusion constants, relative permittivity, and the heat of mixing. The results are also compared to other common sodium chloride force fields.
Lamellar ordering in computer-simulated block copolymer melts by a variety of thermal treatments
Authors: Banaszak M. , Wołoszczuk S. , Jurga S. , Pakula T.
Published: November 2003

A lattice computer simulation of a symmetric A–B–A triblock copolymer melt is reported. This melt is quenched, in simulation, from an athermal state to 39 different temperatures using cooperative motion algorithm. Energy, specific heat, copolymer end-to-end distance, bridging fraction, lamellar spacing, concentration profiles, and microstructure visualizations are reported. The quenching simulation results are compared with those obtained by alternative thermal treatments, that is by slow heating and slow cooling. Quenches yield data consistent with theory and experiment, whereas slow cooling and slow heating results do not capture the expected behavior for the lamellar spacing and the bridging fraction. Finally, at very low temperatures, below the conventional order–disorder transition temperature, an additional ordering is recorded, from a conventional lamellar phase to a lamellar structure showing copolymer junction points condensed into a two-dimensional plane.
A Poisson–Boltzmann dynamics method with nonperiodic boundary condition
Authors: Lu Qiang , Luo Ray
Published: November 2003

We have developed a well-behaved and efficient finite difference Poisson–Boltzmann dynamics method with a nonperiodic boundary condition. This is made possible, in part, by a rather fine grid spacing used for the finite difference treatment of the reaction field interaction. The stability is also made possible by a new dielectric model that is smooth both over time and over space, an important issue in the application of implicit solvents. In addition, the electrostatic focusing technique facilitates the use of an accurate yet efficient nonperiodic boundary condition: boundary grid potentials computed by the sum of potentials from individual grid charges. Finally, the particle–particle particle–mesh technique is adopted in the computation of the Coulombic interaction to balance accuracy and efficiency in simulations of large biomolecules. Preliminary testing shows that the nonperiodic Poisson–Boltzmann dynamics method is numerically stable in trajectories at least 4 ns long. The new model is also fairly efficient: it is comparable to that of the pairwise generalized Born solvent model, making it a strong candidate for dynamics simulations of biomolecules in dilute aqueous solutions. Note that the current treatment of total electrostatic interactions is with no cutoff, which is important for simulations of biomolecules. Rigorous treatment of the Debye–Hückel screening is also possible within the Poisson–Boltzmann framework: its importance is demonstrated by a simulation of a highly charged protein.
Density-functional calculations of the structures, binding energies, and spin multiplicities of Fe–C clusters
Authors: Noya E. G. , Longo R. C. , Gallego L. J.
Published: November 2003

We report ab initio calculations of the structures, binding energies and spin multiplicities of the clusters Fe2, C2, FeCn (n=1–4) and Fe2Cn (n=1–3) using a density-functional method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation for exchange and correlation. Our results for the pure dimers and the monometallic carbide clusters are in good general agreement with those obtained in previous theoretical studies and with available experimental data. All the dimetallic carbide clusters are predicted to have cyclic planar geometries that are stabilized (except, of course, in Fe2C) by transannular bonds. In particular, the pentagonal geometry of Fe2C3, with transannular Fe–Fe and Fe–C bonds and an FeC2 unit that is almost identical to free FeC2, parallels that of Ti2C3. However, this Fe2C3 structure is almost isoenergetic with another in which the C atoms aggregate to form a quasilinear C3 substructure, as in Co2C3. This is consistent with the position of Fe in the 3d metal series, intermediate between met-car formers (Ti, V, Cr) and nonformers (Co, Ni), and with the fact that mass spectra show Fe8C12 not to be significantly more stable than FemCn clusters of several other stoichiometries.
Combined multidimensional anharmonic and parity violating effects in CDBrClF
Authors: Quack Martin , Stohner Jürgen
Published: November 2003

Parity violation causes rovibrational frequency shifts in infrared and microwave spectra between the corresponding lines of enantiomers of chiral molecules. In previous theoretical treatments of this effect simple harmonic and anharmonic adiabatic approximations were used which assumed that the vibrational potential as well as the parity violating potential are separable in normal (or local) coordinates. In the present work we investigate in detail the influence of nonseparable anharmonic couplings on vibrational frequency shifts caused by the parity violating potential in CDBrClF. We use the strongly coupled four-dimensional CD- and CF-chromophore subspaces and discuss how relative frequency shifts are influenced by coupling in the pure vibrational potential as well as in the parity violating potential. A four-dimensional parity violating potential energy hypersurface has been determined ab initio and fitted to a polynomial expansion. We analyze the nonseparable multidimensional representation of the parity violating potential in a chiral molecule. The effects of the multidimensional anharmonic couplings provide the dominant corrections. They are found to be about 20% for the expectation value of the parity violating energy difference ΔpvE between enantiomers (coupled ΔpvE/hc≈1.76×10−12 cm−1 compared to 1.96×10−12 cm−1 uncoupled). The corrections due to anharmonic multidimensional coupling can be more than a factor of 2 for vibrational frequency shifts, depending on the mode considered.
Modeling energy transfer in molecular collisions: Statistical theory versus experiment for highly excited toluene and azulene
Authors: Nilsson Daniel , Nordholm Sture
Published: November 2003

The recent development and application of the method of kinetically controlled selective ionization has produced detailed and reliable data on the collisional energy transfer kernel P(E′,E) entering master equation theories of unimolecular reaction rates. Here we test the ability of our partially ergodic collision theory (PECT) to predict the functional form of the observed kernel leaving only one parameter, the first moment of the distribution 〈ΔE〉, to be input from other sources. The data explored here include two reactant molecules, toluene and azulene, in collisions with 18 and 8 medium molecules, respectively, ranging from inert gas atoms to n-heptane. The initial energy of the reactant molecule is varied from 10 000 cm−1 to 49 000 cm−1 and 30 000 cm−1, respectively. The energy transfer efficiency βE is about one-tenth of its ergodic collision limit of unity. The PECT is found to fit the monoexponential form of the kernel determined from the experimental data over a broad range of initial energies E including tail regions of very low probability. A minor but systematic deviation is observed for nearly elastic collisions of large medium molecules. The functional fit is good enough to effectively allow the three parameters of the monoexponential experimental kernel to be replaced by a single parameter representing energy transfer efficiency.
Chemically induced ferromagnetic spin coupling: Electronic and geometric structures of chromium–oxide cluster anions, Cr2On− (n=1–3), studied by photoelectron spectroscopy
Authors: Tono Kensuke , Terasaki Akira , Ohta Toshiaki , Kondow Tamotsu
Published: November 2003

Dichromium oxide cluster anions, Cr2On− (n=1–3), were found to possess highly spin-polarized electronic structures, which were revealed by the measurements of photoelectron spectra and the analyses by the density-functional calculations. Their spin magnetic moments were as large as 9, 9, and 7 μB for n=1, 2, and 3, respectively, due to a ferromagnetic coupling between local spins on the chromium atoms. The ferromagnetic spin couplings were caused predominantly by a superexchange-type Cr–Cr interaction through an oxygen atom at the bridge site, where a significant mixing of Cr 3d with O 2p orbitals stabilized the ferromagnetic states. The high-spin characters of Cr2On− are in striking contrast to that of a pure chromium dimer, which is known to exhibit an antiferromagnetic spin coupling due to the strong Cr–Cr covalent bond. The present ferromagnetic spin couplings should, therefore, be induced by oxidation. These findings support a concept that a chemical reaction controls magnetic properties of molecules and clusters.
The superfluid transition in helium clusters
Authors: Jortner Joshua
Published: November 2003

We address cluster size effects on the λ temperature (Tλ) for the rounded-off transition for the Bose–Einstein condensation and for the onset of superfluidity in (4He)N clusters of radius R0=aN1/3, where a=3.5 Å is the constituent radius. The phenomenological Ginsburg–Pitaevskii–Sobaynin theory for the order parameter of the second-order phase transition, in conjunction with the free-surface boundary condition, results in a scaling law for the cluster size dependence of Tλ, which is defined by the maximum of the specific heat and/or from the onset of the finite fraction of the superfluid density. This size scaling law (Tλ0−Tλ)/Tλ0∝R0−1/ν∝N−1/3ν, where ν (=0.67) is the critical exponent for the superfluid fraction and for the correlation length for superfluidity in the infinite bulk system, implies the depression of the finite system Tλ relative to the bulk value of Tλ0. The quantum path integral molecular dynamics simulations of Sindzingre, Ceperley, and Klein [Phys. Rev. Lett. 63, 1601 (1989)] for N=64, 128, together with experimental data for specific heat of He4 in porous gold and in other confined systems [J. Yoon and M. H. W. Chan, Phys. Rev. Lett. 78, 4801 (1997); G. M. Zahssenhaus and J. D. Reppy, ibid. 83, 4800 (1999)], are accounted for in terms of the cluster size scaling theory (Tλ0−Tλ)/Tλ0=(πξ0/a)3/2N−1/2, where ξ0=1.7±0.3 Å is the “critical” amplitude for the correlation length in the bulk. The phenomenological theory relates Tλ for the finite system to the correlation length ξ(T) for superfluidity in the infinite bulk system, with the shift (Tλ0−Tλ) being determined by the ratio R0/ξ(T), in accord with the theory of finite-size scaling.
An efficient Monte Carlo method for calculating ab initio transition state theory reaction rates in solution
Authors: Iftimie Radu , Salahub Dennis , Schofield Jeremy
Published: November 2003

In this article, we propose an efficient method for sampling the relevant state space in condensed phase reactions. In the present method, the reaction is described by solving the electronic Schrödinger equation for the solute atoms in the presence of explicit solvent molecules. The sampling algorithm uses a molecular mechanics guiding potential in combination with simulated tempering ideas and allows thorough exploration of the solvent state space in the context of an ab initio calculation even when the dielectric relaxation time of the solvent is long. The method is applied to the study of the double-proton transfer reaction that takes place between a molecule of acetic acid and a molecule of methanol in tetrahydrofuran. It is demonstrated that calculations of rates of chemical transformations occurring in solvents of medium polarity can be performed with an increase in the cpu time of factors ranging from 4 to 15 with respect to gas-phase calculations.
Nonequilibrium molecular-dynamics study of the vibrational energy relaxation of peptides in water
Authors: Nguyen Phuong H. , Stock Gerhard
Published: November 2003

A nonequilibrium description of the vibrational-energy relaxation of solvated flexible molecules such as small peptides in aqueous solution is outlined. Having in mind to employ standard biomolecular molecular-dynamics program packages, several methodological developments are introduced. To calculate the vibrational normal-mode energies for a system undergoing large-amplitude motion, an instantaneous normal-mode analysis is employed. To mimic the laser excitation of a given vibrational mode in its excited states, a computational scheme is proposed which allows us to calculate the nonequilibrium phase-space initial conditions for the solute and the solvent atoms. It is demonstrated that the vibrational relaxation dynamics sensitively depends on the accurate representation of the initially excited normal mode. In particular, effects of the quantum-mechanical zero-point energy contained by the initial state are investigated, thus elucidating the importance of quantum fluctuations. To study the validity and the performance of the method, the laser-induced amide I ν=1→0 energy relaxation of N-methylacetamid in D2O is considered. The vibrational energy relaxation rate obtained from the nonequilibrium simulations is in qualitative agreement with experiment, whereas a Landau–Teller-type calculation underestimates the rate considerably. The virtues and problems of the nonequilibrium description are discussed in some detail.
Enhancement of tracer diffusion in polypropylene glycol
Authors: Wang C. H. , Gong S. S.
Published: November 2003

Tracer diffusion coefficients of camphorquinone in polypropylene glycol (PPG) with molecular weight 400, 1000, 2000, and 4000 have been measured as a function of temperature by using the forced Rayleigh scattering technique. At a fixed temperature, the diffusion coefficient decreases with increasing molecular weight of PPG. A master curve is obtained when the diffusion data are normalized with respect to the viscosity of the PPG liquid. A modified Stokes–Einstein equation for tracer diffusion is used to calculate the effective hydrodynamic radius R*. Below room temperature, R* is found to decrease with decreasing temperature. The decrease of R* is discussed in terms of dynamic heterogeneity and rotation–translation coupling mechanisms. The rotation–translation coupling mechanism has recently been proposed to account for the enhancement of the tracer diffusion coefficient in the molecular liquid in the supercooled state. Comparing R* calculated for CQ in PPG with that for the same tracer in salol in the supercooled state over the same temperature range, one finds that R* in PPG decreases consistently less rapidly than in salol. The result is consistent with the fact that the rotation–translation coupling in PPG is considerably weaker than in liquid salol.
Magnesium: Comparison of density functional theory calculations with electron and x-ray diffraction experiments
Authors: Friis J. , Madsen G. K. H. , Larsen F. K. , Jiang B. , Marthinsen K. , Holmestad R.
Published: November 2003

Accurate experimental structure factors for Mg have been measured and compared with density functional theory (DFT) to test some commonly used functionals and self-interaction correction (SIC) schemes. Low order structure factors, free of extinction and on absolute scale, were measured accurately by quantitative convergent beam electron diffraction. In addition, a complete set of structure factors up to sin θ/λ=1.6 Å−1 was measured by x-ray diffraction at 10 K. The DFT calculations were performed using the full potential linearized augmented plane wave method. It was found that the agreement with experiment increases when going from the local density approximation (LDA) to the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhofer and further to the GGA of Engel and Vosko. Applying the SIC of Perdew and Zunger to the core states for LDA does not improve the agreement with theory, while applying the SIC of Lundin and Eriksson results in a significantly improved agreement. This implies that the main source of error in the LDA functional comes from the description of the core densities. Using the functional which agrees best with experiment, a non-nuclear maximum is established in the calculated electron density of beryllium but not of magnesium.
A theoretical study of the Al–(SiO)1,2 compounds
Authors: Alikhani M. E.
Published: November 2003

Density-functional theory and ab initio methods have been applied to characterize the structural features of Al–(SiO)1,2 compounds. According with previous work [Schnöckel et al., J. Chem. Phys. 112, 1444 (2000)], it has been shown that an Al atom spontaneously reacts with SiO forming a linear O-bonded molecule, AlOSi (2Π). Three minima have been found in the potential-energy surface of Al–(SiO)2: two local minima corresponding to cis–cis and to trans–trans forms with two equivalent SiO fragments arising from addition of a second SiO to AlOSi, and the third one corresponding to the global minimum with a chain structure. It has been shown that the chain structure could be obtained either spontaneously from the reaction between an aluminum atom and a cyclic (SiO)2 dimer or from the isomerization reaction of the cis–cis Al–(SiO)2 structure but requiring, in this case, an activation energy about 10 kcal/mol. Bonding characterization performed within topological frameworks reveals that the interaction in all the (SiO)n and Al–(SiO)n (n=1 and 2) compounds should be considered as a highly polar interaction. Structural comparisons have been made between Al–dicarbonyl and Al–disilonyl on the ground of the topological analysis.
An extended Heisenberg model for conjugated hydrocarbons. II. Kekulé basis
Authors: Wu Jian , Schmalz T. G. , Klein D. J.
Published: November 2003

We investigate the solution of a recently proposed [J. Chem. Phys. 117, 9977 (2002)] improved valence bond model in a basis of Kekulé functions. The model, which has the form of an extended Heisenberg Hamiltonian with next-nearest-neighbor and ring permutation terms, operates in the space of all covalent valence bond structures which grows very rapidly with molecular size. It is found that truncation of the basis to include only the very much smaller number of nearest-neighbor spin pairings, the Kekulé functions, yields better than 98% of the exact energy in most cases. Extensions to excited states are discussed. As an illustration of the applicability of the model to large molecules, we solve it for the ground and first excited Hu singlet state of C60 buckminsterfullerene, and obtain results in good agreement with other theoretical treatments. The model shows promise of being an easily applied semiquantitative valence bond model for large molecules.
A two-dimensional Raman spectroscopic study on the structural changes of a polythiophene film during the cooling process
Authors: Shen Yi , Chen Feng’en , Wu Peiyi , Shi Gaoquan
Published: November 2003

Raman spectra of a polythiophene film measured over a temperature range 298–98 K were analyzed using generalized two-dimensional correlation spectroscopy. In the 1600–1250 cm−1 region, the asynchronicity of the bands indicates the existence of different species in the investigated polythiophene sample. At least four distinct bands at 1485, 1454, 1418, and 1360 cm−1 were identified. Among them, the band at 1454 cm−1, attributed to the neutral species of PTh, is found to be sensitive to the temperature-induced structural changes of the polythiophene film, while the bands of the oxidized species are less sensitive to the temperature change during the cooling process.
Phase corrected higher-order expression for surface hopping transition amplitudes in nonadiabatic scattering problems
Authors: Moody Michael P. , Ding F. , Herman Michael F.
Published: November 2003

A time independent semiclassical surface hopping model was previously derived for the numerical evaluation of the transition amplitudes for multisurface problems. This analysis is revisited and a new method incorporating phase information disregarded by the previous model is derived. The coordinate axis is partitioned into a finite number of intervals. The propagation of the wavefront across an interval is again described by a flux conserving transformation. For the one dimensional case transition amplitudes across many intervals can be evaluated via matrix multiplication of the individual interval transforms. Comparison with exact quantum mechanical calculations show that the results generated by this new model are highly accurate. Further this new approach offers a significant increase in computational efficiency. Accurate results can be obtained in calculations employing larger and hence fewer intervals to partition the system where the original model fails. This increase in efficiency has important implications for multidimensional surface hopping problems.
Electron–phonon interactions in the monoanions of fluoroacenes
Authors: Kato Takashi , Yamabe Tokio
Published: November 2003

Electron–phonon interactions in the monoanions of fluoroacenes such as C6F6 (1f), C10F8 (2f), C14F10 (3f), C18F12 (4f), and C22F14 (5f) are studied, and compared with those in the monoanions of acenes and deutero-acenes. The C–C stretching modes around 1500 cm−1 the most strongly couple to the lowest unoccupied molecular orbitals (LUMO) in fluoroacenes. The estimated total electron–phonon coupling constants (lLUMO) are 0.475, 0.473, 0.350, 0.273, and 0.215 eV for 1f, 2f, 3f, 4f, and 5f, respectively. The lLUMO values for fluoroacenes are much larger than those for acenes and deutero-acenes. Possible superconducting transition temperatures (Tcs) for the monoanions of deutero-acenes and fluoroacenes are estimated to be larger than those for the monoanions of acenes. The transition temperature (Tc) value increases much more significantly by H–F substitution than by H–D substitution in acenes. The lLUMO and Tc values significantly decrease with an increase in molecular size from the monoanions of 1f to 5f. The logarithmically averaged phonon frequencies (ωln) do not significantly change with an increase in molecular size in the monoanions of fluoroacenes. The larger displacements of carbon atoms in the vibronic active modes in fluoroacenes than those in deutero-acenes due to larger atomic mass of fluorine than that of deuterium, and the unchanged properties of the orbital patterns of the LUMO as a consequence of H–F and H–D substitution in acenes, are the main reasons why the lLUMO value increases much more significantly by H–F substitution than by H–D substitution, and the reason why the Tc value increases much more significantly by H–F substitution than by H–D substitution in acenes. That is, the detailed properties of vibronic active modes and the electronic structures in the LUMO as well as the molecular weights are closely related to the lLUMO, ωln, and Tc values in the monoanions of fluoroacenes, deutero-acenes, and acenes.
Negative absolute electron mobility, Joule cooling, and the second law
Authors: Robson R. E. , Petrović Z. Lj. , Raspopović Z. M. , Loffhagen D.
Published: November 2003

A number of recent theoretical investigations of electron motion in attaching gases demonstrate the possibility of a steady-state situation in which the electric current opposes the applied field. This phenomenon, which has been called “negative absolute electron mobility”, implies a Joule cooling effect and an associated negative entropy production, suggesting, at first glance, a possible violation of the second law of thermodynamics. In this article we show that the entropy production has in fact two components, the expected negative contribution due to “Joule cooling,” and an additional positive part arising from “attachment heating.” We insist that the total entropy production be positive, in accordance with the second law, and this has the practical implication that the measurable (“bulk”) electron drift velocity must always be positive, even though the actual average (“flux”) velocity may be negative. We discuss the phenomenon physically and take as a numerical example electrons in Ar/F2 mixtures, using Monte Carlo simulation and approximate momentum transfer theory methods to highlight the distinction between the two types of transport coefficient.
Electrical properties of chemical-solution-derived Bi3.54Nd0.46Ti3O12 ferroelectric thin films
Authors: Wu Di , Xia Yidong , Li Aidong , Liu Zhiguo , Ming Naiben
Published: November 2003

Bi 3.54 Nd 0.46 Ti 3 O 12 (BNdT) thin films were prepared on Pt/TiO2/SiO2/Si substrates by chemical solution deposition. X-ray diffraction measurement indicated the polycrystalline nature of these films. Ferroelectric, dielectric, and leakage current characteristics of Pt/BNdT/Pt capacitors were studied. The remanent polarization and coercive field were 8.5 μC/cm2 and 47 kV/cm, respectively, at 5 V stimulative voltage. The relative permittivity and dissipation factor at 100 kHz were around 510 and below 0.02, respectively. Fatigue free behavior of BNdT thin films was confirmed by essentially constant polarizations during 100 kHz bipolar switching up to 1.4×1010 switches. The dependence of leakage current on dc voltage suggested Schottky and Poole-Frenkel emission behavior below 100 kV/cm, followed by dielectric breakdown.
Interfacial reactions between WNx and poly Si1−xGex films
Authors: Kang Sung Kwan , Min Byoung Gi , Kim Jae Jin , Ko Dae-Hong , Kang Han Byul , Yang Cheol Woong , Cho Mann-Ho
Published: November 2003

The interfacial reactions at the WNx/poly Si interface and at the WNx/poly Si0.4Ge0.6 interface with the annealing conditions were investigated using high resolution transmission electron microscope (HR-TEM) and x-ray photoelectron spectroscopy (XPS). In the WNx/poly Si system, after a short period of annealing, an island-type precipitation was clearly observed under the Si–N layer, which was formed between WNx and poly Si. HR-TEM image and XPS analysis indicated that the island-type precipitation is composed of W, Si, and N. With increasing annealing time, the Si–N layer became thinner and island-type precipitation became continuous layer. However, in the WNx/poly Si0.4Ge0.6 system, the formation of precipitation was obviously prevented after annealing and the island-type precipitation remained between the WNx films and the poly Si1−xGex films in spite of long time annealing. These results are attributed to the fact that the Si in poly Si0.4Ge0.6 reacts preferentially with N, produced in the decomposition of WNx films, due to lower Gibbs free energy of the Si compound than the Ge compound and unreacted Ge atoms accumulate at the WNx/poly Si1−xGex interface.
Density functional theory of nonlinear triplet response properties with applications to phosphorescence
Authors: Tunell Ingvar , Rinkevicius Zilvinas , Vahtras Olav , Sałek Paweł , Helgaker Trygve , Ågren Hans
Published: November 2003

We present density functional response theory generalized to triplet excitations. A method based on an exponential parametrization of the spin-dependent density operator is derived for the evaluation of linear and quadratic response functions for spin-dependent perturbations. The developed methodology is applicable to commonly available functionals, also hybrid functionals including exchange–correlation functionals at the general gradient-approximation level and fractional exact Hartree–Fock exchange. Illustrative calculations are presented for singlet–triplet transition moments and phosphorescence lifetimes, providing numerical data on these quantities for the first time using time-dependent density functional theory.
A Lagrangian study of scalar diffusion in isotropic turbulence with chemical reaction
Authors: Mitarai S. , Riley J. J. , Kosály G.
Published: November 2003

Direct numerical simulations are performed of a single-step, nonpremixed, Arrhenius-type reaction developing in isotropic, incompressible, decaying turbulence, for conditions where flame extinction and re-ignition occur. The Lagrangian characteristics of scalar diffusion, information necessary for modeling approaches such as some implementations of probability density function (PDF) methods, are investigated by tracking fluid particles. Focusing on the mixture fraction and temperature as the scalar variables of interest, fluid particles are characterized as continuously burning or noncontinuously burning based upon their recent time history, and noncontinuously burning particles are further characterized based upon their initial regions relative to the flame zone. The behavior of the mixture fraction and temperature fields is contrasted for the different types of particles characterized. Significant differences among these characterized particles are found, for example, in the unclosed conditional expectations of scalar diffusion appearing in the composition PDF equations.
Hall effect and ionized impurity scattering in Si(1−x)Gex
Authors: Kinsler P. , Wenckebach W. Th.
Published: November 2003

Using Monte Carlo simulations, we demonstrate that including ionized impurity scattering in models of Si(1−x)Gex is vital in order to predict the correct Hall parameters. Our results show good agreement with the experimental data of K. B. Joelsson, Y. Fu, W. X. Ni, and G. V. Hansson, J. Appl. Phys. 81, 1264 (1997).
Near theoretical microwave loss in hot isostatic pressed (hipped) polycrystalline yttrium iron garnet
Authors: Nazarov Alexey V. , Ménard David , Green Jerome J. , Patton Carl E. , Argentina Gil M. , Van Hook H. J.
Published: November 2003

The ferromagnetic resonance (FMR) linewidth, the field dependent effective linewidth, and the parallel pump spin wave linewidth were measured for spheres and disks prepared from a block of hot isostatic pressed (hipped) polycrystalline yttrium iron garnet (YIG). All linewidths as well as static magnetization data indicate close to 100% density. Vibrating sample magnetometer measurements give an average saturation induction 4πMs of 1825 G. The FMR half-power linewidths for the spheres at 9.5 GHz were 13 Oe. Linewidths measured over the 9.5–18 GHz frequency range show a small but distinct drop and agree with Schlömann’s theory of anisotropy-dominated two-magnon scattering for polycrystalline ferrites. The effective linewidth versus field data at 10 GHz show a region of strong absorption that corresponds to the width of the spin wave manifold for low wave numbers and a high field value of about 2 Oe. Parallel pumping measurements give minimum spin wave linewidths of 1.2 and 0.6 Oe at 9 and 16.7 GHz, respectively. The 16.7 GHz spin wave linewidths correspond to half-frequency spin waves at 8.35 GHz. The extrapolated linewidths at zero wave number are about 0.5 Oe and match the established intrinsic linewidths expected for YIG single crystals at 8–9 GHz. The spin wave linewidths increase linearly with wave number and are consistent with a transit time scattering process with scattering lengths that are about ten times greater than the average grain size.
On the effects of assuming flow profiles in nonequilibrium simulations
Authors: Delhommelle Jerome , Petravic J. , Evans Denis J.
Published: November 2003

Atomic simulation methods modelling fluid flows often incorporate in the equations of motion the steady state flow profile predicted by Navier–Stokes equations. We show in this work that this may lead to significant errors such as spurious shear induced ordering, unphysical steady state flow profiles or artificial dampening of thermal motion even at shear rates regarded as low in simulation applications. Our results also suggest that nonequilibrium molecular dynamics coupled with the recently developed configurational thermostat, which makes no assumption at all on the flow profile, provides a much more realistic way to study these phenomena.
Microscopic calculation of the static electric susceptibility of polyethylene
Authors: Eilmes A. , Munn R. W. , Mavrantzas V. G. , Theodorou D. N. , Góra A.
Published: November 2003

The dielectric response of polyethylene is treated by a recent microscopic theory of dielectric response in polymer materials using different simulations of the polymer material structure and representations of the polarizability of the polymer molecules as either localized on individual segments or distributed along the chain. The different variants can all reproduce the experimental value with suitable scaling, and provide an understanding of the length scale on which the dielectric response varies within the polymer material. The approach provides a basis for treating properties such as birefringence and the energetics of charged states in polyethylene.
Langevin equation in momentum space
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: November 2003

The generalized Langevin equation encapsulates in the friction kernel the influence of the environment on the reaction coordinate. We have decomposed the Langevin equation in a Fourier transform representation and analyzed how different Fourier components contribute to the friction kernel. We identified that a long-wavelength contribution to the friction kernel originates from collective solvent motions, and we compared it with contributions coming from short-range scattering of the complex with the surrounding solvent atoms.
Excess chemical potential and entropy for pure fluids
Authors: Bomont Jean-Marc
Published: November 2003

A simple method to calculate the excess chemical potential and the entropy for simple fluids in a thermodynamic manner is proposed. This approach can be used in conjunction with any integral equation.
Investigation of calcium as high performance cathode in small-molecule based organic light-emitting devices
Authors: Lai S. L. , Chan M. Y. , Lee C. S. , Lee S. T.
Published: November 2003

While calcium has a low work function, its detrimental reaction with organic molecules such as tris-(8-hydroxyquinoline) aluminum (Alq3) has hindered its applications as cathode in small-molecule based organic light-emitting devices. In this article, we explore the feasibility of using calcium as a high performance cathode in Alq3 based devices by inserting a thin cesium fluoride layer at the Alq3/Ca interface to suppress the damaging effects of Ca to the organic material. Standard NPB/Alq3 devices were fabricated with various cathodes including Mg:Ag, Ca, and CsF/Ca. The device with the CsF/Ca cathode outperforms those with the other two electrodes in terms of both efficiency and stability.
Nonuniform alloying in Ge(Si)/Si(001) quantum dots
Authors: Lang Christian , Nguyen-Manh Duc , Cockayne David J. H.
Published: November 2003

The composition profile of pyramid shaped Ge(Si)/Si(001) quantum dots has been modeled using a combination of atomistic total energy calculations and a Metropolis Monte Carlo process. The analysis of the non-uniform composition profile has revealed the important, separate roles of the strain energy, the surface energy, and the mixing energy as driving forces of the alloying and segregation process. The surface energy was found to drive a redistribution of Ge into the surface layer of the quantum dot, which was followed by two Si-rich layers. In the vertical direction Si is found to redistribute to the bottom resulting in a Ge-rich apex of the quantum dot. This result is compared to a phenomenological description of the composition profile by Tersoff [Phys. Rev. Lett. 81, 3183 (1998)]. The possibility of a misinterpretation of experimental measurements of the composition profile as a result of the Ge-rich surface layer is discussed.
Do isotropic liquid crystals exist?
Authors: Eringen A. Cemal
Published: November 2003

Our present understanding is that the isotropic state of liquid crystals is a Newtonian fluid. Yet, a fundamental theory indicates that there is an isotropic state with elastic properties. Here, the possible existence of this isotropic state is investigated.
Deposition kinetics of colloidal particles at an interface: Interplay of diffusion and gravity
Authors: Faraudo J. , Bafaluy J. , Senger B. , Voegel J.-C. , Schaaf P.
Published: November 2003

In this paper, we analyze the kinetics of irreversible adsorption of hard spheres from a suspension at rest onto a plane under the influence of diffusion and gravity. We have obtained analytical solutions valid in the low coverage limit of the adsorption kinetics. In order to investigate the adsorption kinetics up to higher coverages, we have also performed nonsequential Brownian dynamics computer simulations. It is shown that the widely employed dimensionless radius R* (or, equivalently, the gravitational Péclet number Pe) cannot alone characterize the relative effect of diffusion and sedimentation in adsorption kinetics. The description of the adsorption process requires the introduction of an additional, independent dimensionless number, Gad, which is a combination of the Péclet number and the bulk volume fraction. The adsorption kinetics is dominated by diffusion for Gad≪1 and by sedimentation for Gad≫1, irrespective of the value of R*. In the case of R*>1 and Gad≫1 the observed kinetics is qualitatively similar to the predictions of the ballistic deposition model, although significant deviations are observed. When Gad≫1, it is also shown that blocking effects due to the interaction with previously adsorbed particles are proportional to the volume fraction so that they can be unobservable until the adsorbing surface is nearly saturated.
A set of molecular models for carbon monoxide and halogenated hydrocarbons
Authors: Stoll Jürgen , Vrabec Jadran , Hasse Hans
Published: November 2003

Molecular models are presented for carbon monoxide and 53 halogenated methane, ethane, and ethene derivatives, among which are important alternative refrigerants. The models are based on the two-center Lennard-Jones plus point dipole or plus point quadrupole pair potentials. The model parameters were adjusted to experimental vapor–liquid equilibria of the pure fluids using a highly efficient procedure. The application of these models to the calculation of vapor–liquid equilibria and homogeneous fluid state points by molecular simulation shows good to excellent agreement with experimental results. The present molecular models describe the vapor pressures in most cases significantly better than models available in the literature. Typical mean relative deviations between simulation results and experiments are 0.5% for the saturated liquid density, 4% for the vapor pressure, and 3% for the enthalpy of vaporization. Due to the compatibility of the presented models, the prediction of vapor–liquid equilibria of mixtures is straightforward.
Multiply subtractive generalized Kramers–Kronig relations: Application on third-harmonic generation susceptibility on polysilane
Authors: Lucarini Valerio , Saarinen Jarkko J. , Peiponen Kai-Erik
Published: November 2003

We present multiply subtractive Kramers–Kronig (MSKK) relations for the moments of arbitrary order harmonic generation susceptibility. Using experimental data on third-harmonic wave from polysilane, we show that singly subtractive Kramers–Kronig (SSKK) relations provide better accuracy of data inversion than the conventional Kramers–Kronig (KK) relations. The fundamental reason is that SSKK and MSKK relations have strictly faster asymptotic decreasing integrands than the conventional KK relations. Therefore SSKK and MSKK relations can provide a reliable optical data inversion procedure based on the use of measured data only.
Thinning and disturbance growth in liquid films mobilized by continuous surfactant delivery
Authors: Fischer Benjamin J. , Troian Sandra M.
Published: November 2003

A generalized linear stability analysis is applied to the case of a thin liquid film propelled to spread by a continuous supply of surfactant. The time-dependent base states for the film thickness and surfactant concentration give rise to a nonautonomous system describing disturbance propagation. As a first approximation, the nonautonomous operator is treated as time independent, thereby reducing the system of equations to a standard eigenvalue problem. For the range of parameters investigated, this modal approximation reveals a band of unstable modes corresponding to the growth of transverse, sinusoidal corrugations. A transient growth analysis of the fully time-dependent system, which requires the solution of an initial value problem, also signals the possibility of large disturbance growth. In both cases, significant amplification of infinitesimal disturbances can be traced to the region of the film most rapidly thinned by Marangoni stresses, which is characterized by large interfacial curvature and a sharp variation in shear stress. In contrast to previous models implementing a finite surfactant source that predict asymptotic stability, large transient growth and asymptotic instability are possible for the case of sustained surfactant release.