bluuebunny/crossref_metadata_2025 huggingface dataset:
Density functional studies of UO22+ and AnF6 (An=U, Np, and Pu) using scalar-relativistic effective core potentials
Authors: Han Young-Kyu , Hirao Kimihiko
Published: July 2002

We have performed one-component relativistic calculations to determine the bond lengths and harmonic vibrational frequencies of UO22+ and AnF6 (An=U, Np, and Pu) using various density functional calculations with relativistic effective core potentials (RECPs) and extended basis sets. The calculated spectroscopic constants using small-core RECPs compare favorably with the available all-electron relativistic results for UO22+ and UF6. The results using the hybrid functionals are in very good agreement with the experimental data for the geometries and vibrational frequencies of the AnF6 systems. The performance of pure gradient-corrected functionals is poor, even worse than that of local density functional for the geometries and vibrational frequencies. For the vibrational frequencies, the hybrid functional calculations give qualitatively the correct order with small quantitative deviations from the experimental data. The local density functional provides reliable frequencies for the stretching modes but underestimates the frequencies for the bending modes. The pure gradient-corrected functionals underestimate the frequencies for all the stretching and bending modes. While the large-core RECP calculations provide short bond lengths and a rather poor description for vibrational frequencies relative to small-core RECP calculations for UO22+ and UF6, they may still be a practical choice of method provided hybrid functionals are also used, especially for larger systems.
Analysis of a dissipative resonant tunneling trap by temperature-dependent Langevin–Schrödinger equations
Authors: Peskin Uri
Published: July 2002

Resonant tunneling of electrons through a dissipative barrier is studied within a generalized spin–boson model. The model accounts for a nonlinear inelastic coupling between the electron and harmonic (bath) nuclear modes inside the barrier. In particular, we study the case in which the nuclear frequencies are similar to the decay rate of the electronic resonance states, as may appear in electron transport through narrow molecular barriers. Two cases are considered. In the case of a single responsive mode numerically exact wave-packet dynamics shows that the inelastic coupling increases the time delay of the tunneling electron in the barrier and the decay process out of the barrier is characterized by nonexponential steps. In the case of a multimode (Ohmic) bath an approximate treatment of the dynamics, based on temperature dependent nonlinear Langevin–Schrödinger equations, implies that the trapped electron wavepacket is fully solvated in the barrier due to irreversible dissipation at low temperatures. An interpretation of the solvation process in terms of the mean-field approximation is given and discussed.
The near ultraviolet spectrum of the FCO radical: Re-assignment of transitions and predissociation of the electronically excited state
Authors: Howie Wendy H. , Lane Ian C. , Orr-Ewing Andrew J.
Published: July 2002

Cavity ring-down spectra of the FCO radical, recorded over the wave number range 29 500–31 600 cm−1 reveal rotational structure of the electronically excited state for the first time. The spectra demonstrate the need for a complete re-assignment of the vibronic features: The rotationally resolved bands are successfully simulated as arising from c-type transitions from the ground X̃ 2A′ state to the linear A19″ component of the Ã 2Π state. The bands are attributed to two overlapping vibrational progressions: one progression involves excitation of the F–C–O bending mode (v3′), the other consists of a combination of v3′ and one quantum of the C–F stretch (v2′). Sharp rotational structure is only observed for sub-bands with K′=0; bands with K′>0 are diffuse, indicating rapid, rotation induced predissociation. Band origins, rotational constants for the excited state, and spectral linewidths have been derived from the K′=0–K″=1 sub-bands. All rotational lines are somewhat broadened and there is evidence of linewidths that increase with N′, and hence an additional rotation-induced predissociation mechanism. Vibrational frequencies and rotational constants are in excellent agreement with the predictions of ab initio calculations by Krossner et al., J. Chem. Phys. 101, 3973 (1994); 101, 3981 (1994). The Ã 2Π(A″)–X̃ 2A′ absorption shows characteristics of a transition between two Renner–Teller components and this interpretation is confirmed by careful examination of the electronic structure of the FCO ground state. Implications for assignments of absorption features at higher energy than the spectral region of the current study are discussed, and comparisons are drawn with the much studied electronic spectroscopy of both the HCO radical and the isoelectronic NO2.
The effect of corrugation on the quantum dynamics of dissociative and diffractive scattering of H2 from Pt(111)
Authors: Pijper E. , Kroes G. J. , Olsen R. A. , Baerends E. J.
Published: July 2002

We present results of two dimensional (2D) and three dimensional (3D) calculations for dissociative and diffractive scattering of H2 from Pt(111), using a potential energy surface obtained from density functional theory (DFT) employing the generalized gradient approximation (GGA) in conjunction with a slab representation of the metal surface. The present study is motivated by the importance of Pt as a hydrogenation catalyst, and by a paradox regarding the amount of corrugation of the H2+Pt(111) potential energy surface (PES). Molecular beam experiments on dissociation of D2 from a Pt(111) surface suggest a rather corrugated PES, which is at odds with results from molecular beam experiments on rotationally inelastic diffraction of HD from Pt(111), where only very little diffraction is found, suggesting a weakly corrugated PES. Results of our 3D calculations for off-normal incidence show that the present 3D model does not obey normal energy scaling, and that parallel motion inhibits dissociation at low collision energies, in agreement with the dissociation experiment. On the other hand, substantial diffraction is found, where the diffraction experiment found almost none. For each impact site considered in the 2D calculations, the computed dynamical barrier height, E0, is substantially lower than the barrier height in the PES, Eb, at that site. Both the 2D and the 3D calculations show a large vibrational enhancement of reaction. These effects are not due to a reduced mass effect, the barrier to dissociation being early, but to a decrease in the force constant of the H2 vibration upon approaching the barrier to dissociative adsorption from the gas phase. The vibrational enhancement computed for H2+Pt(111) was not observed in seeded beam experiments on D2+Pt(111) [A. C. Luntz, J. K. Brown, and M. D. Williams, J. Chem. Phys. 93, 5240 (1990)]. However, an analysis performed here strongly suggests that seeded beam experiments will be unable to observe vibrational enhancement if the dissociation of the molecule in ν=0 proceeds without an energetic threshold, as is the case for H2+Pt(111).
Linear scaling computation of the Fock matrix. V. Hierarchical Cubature for numerical integration of the exchange-correlation matrix
Authors: Challacombe Matt
Published: July 2002

Hierarchical cubature is a new method for achieving linear scaling computation of the exchange-correlation matrix central to Density Functional Theory. Hierarchical cubature combines a k-dimensional generalization of the binary search tree with adaptive numerical integration involving an entirely Cartesian grid. Hierarchical cubature overcomes strong variations in the electron density associated with nuclear cusps through multiresolution rather than spherical-polar coordinate transformations. This unique Cartesian representation allows use of the exact integration error during grid construction, supporting O(log N) range-queries that exploit locality of the Cartesian Gaussian based electron density. Convergence is controlled by τr, which bounds the local integration error of the electron density. An early onset of linear scaling is observed for RB3LYP/6-31G *  *  calculations on water clusters, commencing at (H2O)30 and persisting with decreasing values of τr. Comparison with nuclear weight schemes suggests that the new method is competitive on the basis of grid points per atom. Systematic convergence of the RPBE0/6-31G* Ne2 binding curve is demonstrated with respect to τr.
Investigation of orbital momentum profiles of methylpropane (isobutane) by binary (e,2e) spectroscopy
Authors: Deng J. K. , Li G. Q. , He Y. , Huang J. D. , Deng H. , Wang X. D. , Wang F. , Zhang Y. A. , Ning C. G. , Gao N. F. , Wang Y. , Chen X. J. , Zheng Y.
Published: July 2002

Momentum profiles of the valence orbitals of methylpropane, also known as isobutane (CH3CH(CH3)CH3), have been studied by using a high resolution binary (e,2e) electron momentum spectrometer (EMS), at an impact energy of 1200 eV plus the binding energy, and using symmetric noncoplanar kinematics. The coincidence energy resolution of the EMS spectrometer is 0.95 eV full width at half-maximum. The experimental momentum profiles of the valence orbitals are compared with the theoretical momentum distributions calculated using Hartree–Fock (HF) and density functional theory (DFT) methods with the two basis sets of 6-31G and 6-311++G**. The B3LYP functionals are used for the DFT calculations. In general, the experimental momentum distributions are well described by the HF and DFT calculations. The pole strengths of the main ionization peaks from the orbitals in the inner valence are estimated.
Kinetics of the formation of space-charge field in photorefractive polymers
Authors: Yuan Baohong , Sun Xiudong , Hou Chunfeng , Li Yan , Zhou Zhongxiang , Jiang Yongyuan , Li Chunfei
Published: July 2002

We derive a kinetic differential equation for the formation of space-charge field (SCF) in photorefractive polymers and give its general solution, which is based on Schildkraut’s work. It is found that SCF consists of two exponential decay waves and there are two decay factors corresponding to two response time constants of SCF. Changing the parameters in the solution, we can understand the formation of SCF for different cases. In this article, we discuss a case of no traps and find that both static and transient actions of SCF are determined by the grating wave vector k and total incident intensity Ii.
Time evolution of reactants, intermediates, and products in the vibrational predissociation of Br2⋯Ne: A theoretical study
Authors: Miguel Beatriz , Bastida Adolfo , Zúñiga José , Requena Alberto , Halberstadt Nadine
Published: July 2002

A hybrid quantum/classical simulation of the vibrational predissociation of the Br2⋯Ne cluster in the B state is carried out. The resulting lifetimes and final rovibrational state distributions compare very well with the experimental measurements, as well as with accurate quantum mechanical results. The time-evolution of the reactants, products, and intermediates is analyzed by a kinetic mechanism, comporting three elementary steps: direct vibrational predissociation (VP), intramolecular vibrational redistribution (IVR), and evaporative cooling (EC). The importance of intramolecular vibrational redistribution followed by evaporative cooling relative to direct vibrational predissociation is shown to evolve from 100% of VP for the lowest initial vibrational level v=10 to 53% for the highest one v=27. In the cases where IVR is important, the complexes are shown to explore the whole configuration space, in contrast with the cases where dynamics are governed by direct vibrational predissociation for which the complexes mainly evolve in the region around the T-shaped equilibrium configuration. A time-dependent picosecond experiment is proposed to detect the IVR intermediates, based on their different structure. It consists of exciting the complex with a first laser and probing the intermediates with a second laser to an electronic state with a minimum in the collinear configuration where the initially excited state wave function has no weight. The ground state of the positive ion is proposed as the final state, so that ions are detected. An appreciable population of intermediates is predicted for initial excited levels with v⩾20.
Magnetic field penetration in a long Josephson junction imbedded in a wide stripline
Authors: Franz Andreas , Wallraff Andreas , Ustinov Alexey V.
Published: July 2002

The dependence of the first critical field of long linear and annular Josephson junctions on the width of the surrounding stripline, the so called idle region, is investigated experimentally. The stripline modifies the effective Josephson length λeff in the junction. The experimental data are compared with the theory by Caputo et al. [J. Appl. Phys. 85, 7291 (1999)] and good agreement is found. The dependence of the first critical field on the width of the surrounding stripline can be well explained using the same λeff for both annular and linear junctions.
Chalcogenide glass-based three-dimensional photonic crystals
Authors: Feigel A. , Kotler Z. , Sfez B. , Arsh A. , Klebanov M. , Lyubin V.
Published: July 2002

AsSeTe chalcogenide glasses are materials that are photosensitive and have a large refractive index. These properties make these glasses particularly suitable for the fabrication of photonic crystals. We present a way to build three-dimensional photonic structures from chalcogenide glasses using vapor deposition and direct holographic writing. We show that this technique is intrinsically self-aligned, providing a simple way to build layer-by-layer photonic crystals and a four-layer structure demonstrating the principle of the technique.
Controlling electron transfer in strong time-dependent fields: Theory beyond the Golden Rule approximation
Authors: Hartmann Ludwig , Goychuk Igor , Hänggi Peter
Published: July 2002

In this work we apply a generalized Zusman model to study the influence of an external periodic electric field on the dynamics of electron transfer (ET) reactions coupled to an overdamped reaction coordinate which is treated semiclassically. Being nonperturbative in the tunneling coupling this approach goes beyond the conventional Golden Rule description and includes both adiabatic and nonadiabatic electron transfer regimes. Explicit expressions for the ET rates are derived in the high-frequency driving regime and compared with exact numerical results. Our novel analytical findings constitute a useful approximation scheme, as long as the dynamics can be characterized by a single exponential relaxation. We further demonstrate that the Golden Rule description becomes drastically improved in the presence of strong, fast oscillating fields. Moreover, we discuss interesting phenomena such as an inversion of populations and a driving induced transition from an adiabatic to a nonadiabatic reaction dynamics.
GaAsN interband transitions involving localized and extended states probed by resonant Raman scattering and spectroscopic ellipsometry
Authors: Wagner J. , Köhler K. , Ganser P. , Herres N.
Published: July 2002

Interband transitions in GaAs1−xNx (0⩽x⩽0.033), involving a localized resonant N-derived energy level and the GaAs-like L conduction and valence band states, have been studied by resonant Raman scattering and spectroscopic ellipsometry, respectively. Raman scattering by the GaN-like LO2 phonon showed for x≈0.01 a pronounced resonant enhancement for incident photon energies approaching the mostly N-related E+ transition at around 1.8 eV, but not at the E1 and E1+Δ1 interband transitions, reflecting the strongly localized nature of both the N-related electronic level and the Ga–N vibrational mode. Spectroscopic ellipsometry, in contrast, being sensitive to the overall dielectric function of the GaAsN, revealed the effect of N incorporation on the GaAs-like E1 and E1+Δ1 interband transitions, which is a high-energy shift with increasing N-content accompanied by a significant broadening.
On the stability of drift wave spectra with respect to zonal flow excitation
Authors: Malkov M. A. , Diamond P. H. , Smolyakov A.
Published: July 2002

A simple criterion that allows one to determine whether or not a given wave spectrum will generate zonal flows, is derived and analyzed. In the context of a coupled drift wave–zonal turbulence, the results are pertinent to the limit of small zonal flow damping, γd→0, in which previous analyses found that the turbulence vanishes. However, the practically important issue of the drift wave amplitude threshold for zonal flow excitation was not resolved. In its formal mathematical appearance, the criterion obtained is similar to the well-known Penrose criterion that is used for stability analysis of stellar distributions and particle distributions in plasmas. By contrast, the derived criterion, being applied to wave quanta rather than to particle distribution, shows that even “normal” (wave density decaying with wave number) distributions with an intensity above the threshold should generate zonal flows. This clearly points at the ubiquity of the latter.
Atomic scale characterization of oxygen vacancy segregation at SrTiO3 grain boundaries
Authors: Klie R. F. , Browning N. D.
Published: July 2002

We have examined the atomic structure, composition, and bonding at a nominally undoped 58° [001] tilt grain boundary in SrTiO3 in order to develop an understanding of the control that the grain boundary plane exerts over the bulk properties. Room temperature and in situ heating experiments show that there is a segregation of oxygen vacancies to the grain boundary that is increased at elevated temperatures and is independent of the cation arrangement. These measurements provide direct support for recent experimental and theoretical predictions that nonstoichiometry, and in particular oxygen vacancies, are responsible for the widely observed grain boundary properties.
Development of an efficient linear response approach to the Hilbert space multi-reference coupled-cluster theory
Authors: Shamasundar K. R. , Pal Sourav
Published: July 2002

In this paper, we use an analytic linear response to develop efficient expressions for calculating a first-order energy response using the multi-reference Hilbert space coupled-cluster (HSMRCC) theory. Equations for the first-order response are derived and their diagrammatic evaluation is outlined. The Z-vector formalism used in SRCC to eliminate the explicit presence of a cluster amplitude response in favor of a de-excitation operator is generalized to HSMRCC and applied here. We also discuss several aspects of the Z-vector and outline different ways of introducing the technique and appropriateness of these in various circumstances. Efficient expressions for the energy response in terms of state-dependent effective CC density matrices are presented. We also compare our approach with Szalay’s approach based on the generalized Hellmann–Feynmann theorem and discuss the advantages of our approach.
Development of a microstrip gas chamber to study the effect of drift gap on its performance
Authors: Radhakrishna V. , Rajanna K. , Desai S. S. , Shaikh A. M.
Published: July 2002

We present a new microstrip gas chamber design and a detailed experimental study of the effect of drift gap on its performance. The chamber has the provision to vary the drift gap in situ which enables one to carry out the performance study without altering the other set operating parameters such as gas pressure, anode voltage, and drift voltage. The effect of drift gap on detector gain and energy resolution is discussed here. The importance of optimization of drift gap is highlighted. The maximum gain obtained with P10 (Ar90%+CH410%) gas is ∼750 and the best resolution achieved is ∼19% for 5.9 keV x rays.
Thermal motion effect on the filamentation of a strongly collisional current-driven plasma
Authors: Shokri B. , Vazifehshenas T.
Published: July 2002

The effect of thermal motion of charged particles in the filamentation of a nonrelativistic current-driven plasma in the diffusion frequency region is investigated. The period and the establishment time of the filamentation structure and the threshold for instability development are obtained.
Analysis of a novel flexure hinge with three degrees of freedom
Authors: Hsiao Feng-Zone , Lin Tai-Wu
Published: July 2002

The flexure hinge is widely used as the mechanism for a high precision positioning stage with a micrometer or nanometer resolution. In this article we propose a novel flexure hinge with three degrees of freedom in which the motions are restricted in the same plane. An analysis model is developed to analyze the flexure hinge. The results obtained are consistent with those of the finite element method. A characteristic study using the proposed model shows that this flexure hinge has a feature of linear load displacement. In addition, the most effective way to change the stiffness of this flexure hinge is to modify the notch’s radius. A prototype of this novel flexure hinge has been manufactured and the measured characteristics prove the advantages of this proposed model.
Angular trapping and rotational dissociation of a diatomic molecule in an optical centrifuge
Authors: Spanner M. , Ivanov M. Yu.
Published: July 2002

We perform a detailed quantum study of forced molecular rotation in an optical centrifuge, recently proposed by J. Karczmarek [Phys, Rev. Lett. 82, 3420 (1999)]. The approach uses strong nonresonant laser fields with chirped frequency to induce efficient rotational excitation of anisotropic molecules via a sequence of Raman transitions. Quantum calculations firstly of angular confinement (angular trapping) of a molecule in the early stages of the centrifuge evolution and secondly of the resulting rotational dissociation process are carried out herein. The trapping calculations include both angular degrees of freedom while the dissociation calculations include one vibrational and one rotation degree of freedom. Diatomic Cl2 is used as a test case. An extension of the scheme outlined by Karczmarek et al. is proposed as a method of producing molecules in a single selected J=Jz level.
Laser heated diamond cell system at the Advanced Photon Source for in situ x-ray measurements at high pressure and temperature
Authors: Shen Guoyin , Rivers Mark L. , Wang Yanbin , Sutton Stephen R.
Published: July 2002

We describe a laser heated diamond anvil cell system at the GeoSoilEnviroCARS sector at the Advanced Photon Source. The system can be used for in situ x-ray measurements at simultaneously ultrahigh pressures (to >150 GPa) and ultrahigh temperatures (to >4000 K). Design goals of the laser heating system include generation of a large heating volume compared to the x-ray beam size, minimization of the sample temperature gradients both radially and axially in the diamond anvil cell, and maximization of heating stability. The system is based on double-sided laser heating technique and consists of two Nd:YLF lasers with one operating in TEM00 mode and the other in TEM01* mode, optics to heat the sample from both sides, and two spectroradiometric systems for temperature measurements on both sides. When combined with an x-ray microbeam (3–10 μm) technique, a temperature variation of less than 50 K can be achieved within an x-ray sampled region for longer than 10 min. The system has been used to obtain in situ structural data and high temperature equations of state on metals, oxides, and silicates to 3500 K and 160 GPa.
Classical analogs of quasifree quantum stochastic processes given by stochastic states of the quantized electromagnetic field
Authors: Hertfelder C. , Kümmerer B.
Published: July 2002

The mathematical model describing a light beam prepared in an arbitrary quantum optical state is a quasifree quantum stochastic process on the C* algebra of the canonical commutatation relations. For such quantum stochastic processes the concept of stochastic states is introduced. Stochastic quantum states have a classical analog in the following sense: If the light beam is prepared in a stochastic state, one can construct a generalized classical stochastic process, such that the distributions of the quantum observables and the classical random variables coincide. A sufficient algebraic condition for the stochasticity of a quantum state is formulated. The introduced formalism generalizes the Wigner representation from a single field mode to a continuum of modes. For the special case of a single field mode the stochasticity condition provides a new criterion for the positivity of the Wigner function related to the given state. As an example the quantized eletromagnetic field in empty space at temperature T=0 is discussed. It turns out that the corresponding classical stochastic process is not a white noise but a colored noise with a linearly increasing spectrum.
Resolution of molecular polarizabilities of CH3–X and CH3–CH2–X derivatives into atomic terms
Authors: Zitto M. E. , Caputo M. C. , Ferraro M. B. , Lazzeretti P.
Published: July 2002

An additive scheme for resolving average molecular electric dipole polarizabilities into atomic contributions, based on the acceleration gauge for the electric dipole, has been applied to a series of methyl and ethyl derivatives, CH3–X and CH3–CH2–X. Extended calculations have been carried out to test the reliability of the partition method. Gross atomic isotropic contributions have been evaluated for carbon, hydrogen, and heteroatoms, showing a good degree of transferability from molecule to molecule. The theoretical values of atomic polarizabilities can be used to predict fairly accurate average polarizabilities of higher homologous molecules in the series of X-substituted alkanes.
Perturbative corrections to coupled-cluster and equation-of-motion coupled-cluster energies: A determinantal analysis
Authors: Hirata So , Nooijen Marcel , Grabowski Ireneusz , Bartlett Rodney J.
Published: July 2002

We develop a combined coupled-cluster (CC) or equation-of-motion coupled-cluster (EOM-CC) theory and Rayleigh–Schrödinger perturbation theory on the basis of a perturbation expansion of the similarity-transformed Hamiltonian H̄=exp(−T)H exp(T). This theory generates a series of perturbative corrections to any of the complete CC or EOM-CC models and hence a hierarchy of the methods designated by CC(m)PT(n) or EOM-CC(m)PT(n). These methods systematically approach full configuration interaction (FCI) as the perturbation order (n) increases and/or as the cluster and linear excitation operators become closer to complete (m increases), while maintaining the orbital-invariance property and size extensivity of CC at any perturbation order, but not the size intensivity of EOM-CC. We implement the entire hierarchy of CC(m)PT(n) and EOM-CC(m)PT(n) into a determinantal program capable of computing their energies and wave functions for any given pair of m and n. With this program, we perform CC(m)PT(n) and EOM-CC(m)PT(n) calculations of the ground-state energies and vertical excitation energies of selected small molecules for all possible values of m and 0⩽n⩽5. When the Hartree–Fock determinant is dominant in the FCI wave function, the second-order correction to CCSD [CC(2)PT(2)] reduces the differences in the ground-state energy between CCSD and FCI by more than a factor of 10, and thereby significantly outperforms CCSD(T) or even CCSDT. The third-order correction to CCSD [CC(2)PT(3)] further diminishes the energy difference between CC(2)PT(2) and FCI and its performance parallels that of some CCSD(TQ) models. CC(m)PT(n) for the ground state with some multideterminantal character and EOM-CC(m)PT(n) for the excitation energies, however, appear to be rather slowly convergent with respect to n.
Absorption and emission in pinwheel aggregates of oligo-phenylene vinylene molecules
Authors: Spano Frank C.
Published: July 2002

The effects of exciton-vibrational coupling and point defects on the absorption and emission of distyrylbenzene nanoaggregates are treated theoretically. Two aggregate types based on a two-dimensional array of cyclic tetramers (pinwheels) are considered: type A aggregates, composed of chiral pinwheels, and type B aggregates, composed of achiral pinwheels. The low-energy vibronic features in the experimental excitation spectrum arise from vibrationally dressed K=(0,0) excitons, while the more intense blue shifted H-band is due to nearly free K=(0,0) excitons. The K=(0,0) features are polarized primarily along the herringbone plane normal. The lowest Davydov component is polarized in the herringbone plane and is due to the lowest energy K=(π,π) exciton. This state is also responsible for the aggregate emission. The 0-v peaks for v>0 are mainly due to indirect transitions to the ground electronic state containing v phonons, with wave vector sum equal to (π,π). These peaks are largely independent of defect fraction and are polarized primarily along the herringbone plane normal. In stark contrast, the 0–0 emission critically depends on the concentration of point defects and is polarized entirely in the herringbone plane. This wavelength dependent emission polarization is in full agreement with experimental observations. Type A aggregates are weakly emissive, with the 0–0 emission peak vanishing in defect-free aggregates and increasing with defect concentration. The reverse holds for type B aggregates: the 0–0 intensity scales with the number of molecules in the aggregate and decreases with defect concentration. Sufficiently large type B aggregates are superradiant, and may be used to enhance the quantum yield in optical devices such as light-emitting diodes.
Photocurrent mapping with submicron resolution on the silicon-electrolyte junction by using near-field optics
Authors: Diesinger Heinrich , Bsiesy Ahmad , Hérino Roland , Huant Serge
Published: July 2002

We have developed a technique allowing photocurrent (PC) mapping of silicon surface in contact with an electrolyte which offers an unprecedented spatial resolution. The photocurrent is generated by near-field optics using an optical tip with a 100 nm diameter aperture as an illumination source. The comparison between topographic and photocurrent mapping of SiO2/Si mesas is used to demonstrate the feasibility of such a technique. Topographic and PC images show 300 nm lateral resolution. It is shown that this resolution is topography limited, i.e., determined by the tip-topography interaction. Indeed, PC mapping on topography-less patterned porous silicon/silicon samples shows that the lateral resolution can be as good as 100 nm, limited by the aperture size.
The inelastic neutron scattering of two benzylic amide [2]catenanes
Authors: Leigh David A. , Parker Stewart F. , Timpel Dirk , Zerbetto Francesco
Published: July 2002

The inelastic neutron scattering (INS), spectra of the isophthaloyl [2]catenane and the sebacoyl [2]catenane are measured and compared with simulated spectra obtained by molecular dynamics (MD), based on a molecular mechanics model (MM), that has been extensively used before to treat this class of molecules. Both the isolated molecule approximation (IMA), and the crystal approach, implemented through boundary conditions, are used. Good agreement with the experiments is met for the spectra of the two catenanes for the location of the most prominent bands. The crystal calculations give better intensity distributions than the IMA. Further analysis of the simulations shows that the combination of INS spectroscopy and MD is well suited to assess the strength of intramolecular and intermolecular hydrogen bonds that are the crucial interactions that govern the dynamics of these systems.
Dynamics of ice layers deposited on MgO(001): Quasielastic neutron scattering experiments and molecular dynamics simulations
Authors: Toubin C. , Picaud S. , Hoang P. N. M. , Girardet C. , Demirdjian B. , Ferry D. , Suzanne J.
Published: July 2002

The dynamical behavior of a thin film of ice Ih deposited on MgO(001) surface has been investigated both experimentally and theoretically. Incoherent neutron quasielastic scattering experiments, using uniform MgO powders, show that a quasiliquid water layer of monolayer thickness exists at T=265 K. The translational mobility of this layer, with a diffusion coefficient Dt=1.5×10−5 cm2 s−1, is close to that of liquid water. At T=270 K, the thickness of the quasiliquid layer increases to about two layers, showing no appreciable change in the Dt value but an increase of the rotational mobility from 6×109 s−1 to 1.2×1010 s−1. Classical molecular dynamics simulations are performed to determine the translational and orientational order parameters and diffusion coefficients of the supported ice film as a function of temperature within 190 and 270 K, and to compare the results with those obtained for bulk ice. It is shown that the whole supported ice film is much more disordered than bulk ice, with melting temperature around 235 K for the TIP4P potential used, while the melting temperatures of the outermost layer are nearly the same (around 220 K) for the supported film and bulk ice. Comparison of the values of the translational and orientational diffusion coefficients obtained in simulation and experiments displays a good agreement. Although the calculated value of the surface melting temperature is underestimated by 15% with respect to the experimental result, the present study indicates clearly the influence of the support on the melting process.
Direct detection of the magnetic flux noise from moving vortices in wide YBa2Cu3O7−δ grain boundary junctions
Authors: Hirano S. , Oyama H. , Kuriki S. , Morooka T. , Nakayama S.
Published: July 2002

We measured the flux of a wide bicrystal grain boundary of YBa2Cu3O7−δ film, cooled to 77 K in a field of 10 μT, using a superconducting thin-film coil and Nb-based superconducting quantum interference device (SQUID). When the applied field was changed to, and above, a small threshold value, the sample showed random switching noises with a sharp increase in low-frequency noise power. The results suggested that there was long-distance movement of vortices over 20–500 μm within the grain boundary, driven by the shielding current. The vortex movement was suppressed to lengths of less than 1 μm in a slotted grain boundary by making an array of SQUIDs that can hold vortices stably.
Non-s-wave behavior of c-axis aligned Pr1.85Ce0.15CuO3.97 superconducting powder
Authors: Chiang H. C. , Hsu Y. Y. , Chang B. C. , Lin B. N. , Hsu T. I. , Ku H. C.
Published: July 2002

The tetragonal c-axis alignment of Tc=23 K Pr1.85Ce0.15CuO3.97 superconducting powder was achieved in a 1 T alignment field with rotating powder/epoxy holder perpendicular to the alignment field. The in-plane penetration depth λab for this electron-doped superconductor was derived using zero-field-cooled Meissner diamagnetic data with low applied field (Ba<Hc1ab) parallel to the c-axis aligned powder. A small Pr paramagnetic contribution was subtracted. The temperature dependence of Δλab(T)=λab(T)−λab(0) [with λab(0)∼133 nm] up to ∼0.5 Tc can be fitted well with a non-s-wave or d-wave-like power law Δλab(T)=AT2+CT4, where the T2 law is dominated at low temperature. The present result agrees with recent phase-sensitive experiment, which indicates d-wave nature for these electron-doped 214 cuprates.
Influence of alpha particle bombardment and postannealing on photoluminescence from GaAs/Al0.35Ga0.65As multiple quantum wells
Authors: Kundrotas J. , Dargys A. , Valušis G. , Ašmontas S. , Köhler K. , Leroy C.
Published: July 2002

Multiple quantum well (MQW) samples grown by the molecular beam epitaxy method were irradiated by alpha particles from isotope Pu239. The photoluminescence (PL) spectra and PL integrated intensity dependencies are presented at various alpha particle fluences, up to 1011 cm−2. The experimental results are in agreement with a model which assumes that point centers (residual impurities and point defects introduced during irradiation) are responsible for PL intensity decrease with the alpha particle fluence. It was found that annealing of irradiated MQW samples at a temperature above 650 K nearly restores the PL intensity. An enhancement of PL by more than an order of magnitude was observed at annealing temperatures higher than 850 K, just before GaAs and Al0.35Ga0.65As interdiffusion begins.
Modeling the effect of grain size and dislocation density on hysteretic magnetic properties in steels
Authors: Sablik M. J.
Published: July 2002

In this article, the Jiles–Atherton model is modified to elucidate the variation of magnetic properties with grain size and dislocation density. The modified model predicts generally that coercive field increases with increasing dislocation density and inverse grain size and that remanent flux density and permeability at the coercive field decrease with increasing dislocation density and inverse grain size. Using the same model, it should be possible to model the effects of grain size and dislocation density on other magnetic measurements such as nonlinear harmonics of the flux density or Barkhausen noise.
Magnetic force microscopy studies of the domain structure of Co/Pd multilayers in a magnetic field
Authors: Rushforth A. W. , Main P. C. , Gallagher B. L. , Marrows C. H. , Hickey B. J. , Dahlberg E. D. , Eames P.
Published: July 2002

We have measured the magnetic domain patterns in Co/Pd multilayers of varying thickness using magnetic force microscopy in the presence of an external magnetic field applied perpendicular to the multilayers. We find that the domain patterns evolution is in qualitative agreement with existing theories for single layer thin films. Our results are in reasonable agreement with a theoretical model of domains appropriate to multilayer films.
Spin filtering in a magnetic–electric barrier structure
Authors: Papp G. , Peeters F. M.
Published: July 2002

The spin-dependent tunnelling of two-dimensional electrons through a magnetic barrier can be substantially enhanced by the addition of an electric barrier. The spin polarization is found to be strongly dependent on the incident wave vector parallel to the barrier, the incident electron energy, and the height of the electric barrier. The conductance for the spin-up and spin-down electrons can be tuned with this electrical barrier.
Demixing and the force between parallel plates immersed in binary liquid mixtures
Authors: Greberg H. , Patey G. N.
Published: July 2002

Grand Canonical Monte Carlo calculations are used to examine the phase behavior of model binary liquid mixtures confined between parallel plates. The stable and metastable states are identified by direct evaluation of the grand potential. It is shown that demixing transitions can lead to long-range attractive forces acting between the plates, verifying earlier predictions based on approximate integral equation theories. The nature of these transitions and the physical origin of the accompanying attractive forces are discussed.
Magnetostriction and a magnetoelastic mechanism for the irreversible behavior of the magnetoresistance in layered antiferromagnets
Authors: Gomonay E. V. , Loktev V. M.
Published: July 2002

A theoretical model is proposed for describing the irreversible behavior of the magnetoresistance of underdoped cuprates RBa2Cu3Ox in an external magnetic field over a wide range of temperatures. The underlying assumption of the model is a magnetoelastic mechanism for the formation of the domain structure of the collinear antiferromagnet owing to long-range “stray” fields resulting from the difference in the elastic properties of the bulk and surface. The law of corresponding states in reduced variables is established for the magnetoresistance, external magnetic field, and temperature, and the temperature dependence of the “monodomainization” field, the field at which the sample is brought to a single-domain state, is determined. The theoretical relations obtained give a comprehensive description of the features of the magnetoresistance of RBa2Cu3Ox and are in quantitative agreement with the experimental data.
Optical patterning and photochemical fixation of polymer nanoparticles on glass substrates
Authors: Ito Syoji , Yoshikawa Hiroyuki , Masuhara Hiroshi
Published: July 2002

A method for fixing patterned nanoparticles onto a substrate was developed by combining photopolymerization with a laser manipulation technique. Nanoparticles were dispersed in ethylene glycol containing monomer, crosslinker, and photoinitiator, and gathered at the focal point of a trapping laser beam (1064 nm) just on a glass substrate. Local photopolymerization within and around the nanoparticles assembly was induced by additional irradiation of a pulsed-laser beam (355 nm), resulting in generation of polyacrylamide containing nanoparticles. The polymerized assembly was evaluated by atomic force microscope observation. By scanning both trapping and excitation laser beams, patterned nanoparticles could also be fixed on a glass substrate.
Coupled catalytic oscillators: Beyond the mass-action law
Authors: Zhdanov V. P. , Kasemo B.
Published: July 2002

We present Monte Carlo simulations of the reaction kinetics corresponding to two coupled catalytic oscillators in the case when oscillations result from the interplay between the reaction steps and adsorbate-induced surface restructuring. The model used is aimed to mimic oscillations on a single nm catalyst particle with two kinds of facets or on two catalyst particles on a support. Specifically, we treat the NO reduction by H2 on a composite catalyst containing two catalytically active Pt(100) parts connected by an inactive link. The catalyst is represented by a rectangular fragment of a square lattice. The left- and right-hand parts of the lattice mimic Pt(100). With an appropriate choice of the model parameters, these sublattices play a role of catalytic oscillators. The central catalytically inactive sublattice is considered to be able only to adsorb NO reversibly and can be viewed as a Pt(111) facet or a support. The interplay of the reactions running on the catalytically active areas occurs via NO diffusion over the boundaries between the sublattices. Using this model, we show that the coupling of the catalytically active sublattices may synchronize nearly harmonic oscillations observed on these sublattices and also may result in the appearance of aperiodic partly synchronized oscillations. The spatio-temporal patterns corresponding to these regimes are nontrivial. In particular, the model predicts that, due to phase separation, the reaction may be accompanied by the formation of narrow NO-covered zones on the left and right sublattices near the boundaries between these sublattices and the central sublattice. Such patterns cannot be obtained by using the conventional mean-field reaction-diffusion equations based on the mass-action law. The experimental opportunities to observe the predicted phenomena are briefly discussed.
Magnetoresistance of manganite thin films induced by reaction with substrate
Authors: Hamaya K. , Taniyama T. , Yamazaki Y.
Published: July 2002

Enhanced low-field magnetoresistance (MR) of polycrystalline lanthanum strontium manganite thin films is demonstrated using an approach in which effective barriers are formed by the heat-treatment induced reaction with an insulating substrate. The heat treatment induces two chemically different processes in the film, i.e., grain growth and chemically reactive penetration. In the former process, the MR decreases due to reduction of the number of grain boundaries, while in the latter the penetration of an insulating substrate and reacted materials forms effective barriers for the electron tunneling between ferromagnetic grains. The formation of effective barriers causes a factor of 3 larger MR than as-prepared films. These results suggest that the MR can be prospectively controlled in this fashion.
Two-dimensional vibrational spectroscopy. VIII. Infrared optical Kerr effect and two-color infrared pump–probe measurements
Authors: Cho Minhaeng
Published: July 2002

The infrared optical Kerr effect (IR-OKE) and two-color infrared pump–probe spectroscopies, which can be directly used to study intermolecular vibrational energy relaxation pathways, are theoretically studied. The IR-OKE method involves an intense IR pulse with controlled frequency to build up large population on a specific intramolecular vibrationally excited state. After a finite mixing time, which is also experimentally controlled, the time evolution of the nonequilibrium density matrix is detected by using the femtosecond optical Kerr effect measurement. Due to the nonequilibrium population distribution initially induced by the intensive infrared pulse, the measured OKE signal differs from that measured for a molecular system initially in thermal equilibrium state. Particularly, it is found that the deviation from the harmonic response can be specifically measured by using this method. By following the same procedure developed in this paper, the two-color IR pump–probe absorption spectroscopy is also considered and the corresponding nonequilibrium IR response function is theoretically investigated with a direct comparison with complementary IR-OKE response function. By using the perturbation theory, the corresponding response functions are obtained in terms of the molecular properties such as linear and nonlinear spatial derivatives of dipole and polarizability and cubic and quartic anharmonic coefficients. Also, it is emphasized that this method is conceptually quite similar to the transient nuclear Overhauser effect nuclear magnetic resonance spectroscopy.
Formation of cobalt silicide spikes in 0.18 μm complementary metal oxide semiconductor process
Authors: Dai J. Y. , Guo Z. R. , Tee S. F. , Tay C. L. , Er Eddie , Redkar S.
Published: July 2002

Co silicide spikes have been found in active contact salicidation in complementary metal oxide semiconductor devices during failure analysis by means of transmission electron microscopy examination. Scanning transmission electron microscopy, energy dispersive x-ray analysis and microdiffraction study revealed that these spikes are CoSi2 with an epitaxial relationship with Si of (111)CoSi2//(111)Si and [11̄0]CoSi2//[11̄0]Si. The formation of the CoSi2 spikes are suspected to be due to the presence of undesired SiOx residue between Co film and Si substrate which acts as a solid diffusion membrane to cause the Si rich phase CoSi2 to precipitate directly inside Si lattice.
A 2 kHz–100 MHz dynamic amplifier for tracking targets of variable amplitude
Authors: Jain Mahaveer K. , Bitler J. Samuel , Grimes Craig A.
Published: July 2002

A 2 kHz–100 MHz constant-output dynamic amplifier design is presented. Application of the amplifier is shown for tracking wireless magnetoelastic and magnetoacoustic environmental sensors [C. A. Grimes et al., Rev. Sci. Instrum. 70, 4711 (1999); K. Loiselle and C. A. Grimes, Rev. Sci. Instrum. 71, 1141 (2000)], the detected amplitude which varies significantly with spatial location from the detector. The amplification of the dynamic amplifier, 80 dB, is controlled by inverted feedback voltage; the design is suitable for an input voltage range of −150 to −10 dB. The dynamic amplifier enhances the detection range of magnetoacoustic sensors by an approximate factor of three, and that of magnetoelastic sensors by a factor of 1.25, with the magnetoelastic sensors having comparatively less detection range increase due to higher background noise levels.
Encounter-controlled reactions between interacting walkers in finite lattices: Complex kinetics and many-body effects
Authors: Nicolis C. , Kozak John J. , Nicolis G.
Published: July 2002

Montroll’s approach to diffusion-controlled annihilation reactions recently generalized by the present authors to account for the simultaneous displacement of two walkers, is extended by including more complex kinetic schemes and many-body effects. The mean walklength to reaction and the spatial organization of the reactants in a finite planar lattice is evaluated analytically and by Monte Carlo simulations in two representative schemes involving, respectively, a single autocatalytic reaction and an autocatalytic reaction coupled to isomerization. While in the first scheme the results are in qualitative (though not quantitative) accord with mean-field predictions, marked qualitative differences with mean-field behavior are found in the second scheme.