bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoelectron diffraction at the surface of amorphous carbon nitride
Authors: Ripalda J. M. , Garcı́a de Abajo F. J. , Montero I. , Galán L. , Van Hove M. A.
Published: July 2002

The line shape of N 1s photoemission spectra in a certain type of amorphous carbon nitride is shown to depend strongly on the polar angle of photoelectron emission. This effect is explained in terms of photoelectron forward scattering due to preferential orientation of bonds near the surface. An alternative model based on different indepth distributions for each of the nitrogen chemical species does not provide a satisfactory explanation of the observed angular dependence. The changes with polar emission angle of the photoelectron and Auger electron spectra are compared to elucidate the nature of each of the N 1s photoemission peaks.
Increased electrical activation in the near-surface region of sulfur and nitrogen coimplanted GaAs
Authors: Yu K. M. , Walukiewicz W. , Shan W. , Wu J. , Beeman J. W. , Ager J. W. , Haller E. E.
Published: July 2002

A large increase in the electrical activation of sulfur coimplanted with nitrogen in GaAs is observed within a thin (<500 Å) near-surface region. A free electron concentration as high as 1.5×1019 cm−3 is observed in this layer, which is a factor of 5 higher than in a GaAs sample implanted with S only. The high free electron concentration in this thin layer is the result of the incorporation of N on the As site which forms a thin dilute GaNxAs1−x alloy layer. The increased electrical activity of S in this layer is explained by the N-induced modifications of the conduction band as predicted by the band anticrossing model. The measured free electron concentration is consistent with a GaNxAs1−x alloy layer with x∼0.3%. The results have important practical implications on the fabrication of low-resistance, nonalloyed ohmic contacts to n-type GaAs.
Bulk-quantity Si nanosphere chains prepared from semi-infinite length Si nanowires
Authors: Peng H. Y. , Wang N. , Shi W. S. , Zhang Y. F. , Lee C. S. , Lee S. T.
Published: July 2002

Bulk-quantity Si nanosphere chains have been fabricated. This is accomplished via the spheroidization of Si nanowires of semi-infinite lengths. The process has been extensively investigated by transmission electron microscopy. The nanosphere chains consisted of equally spaced Si crystalline nanospheres connected by Si-oxide bars. The transition from Si nanowires to Si nanosphere chains was determined by the annealing temperature, ambient pressure, initial Si nanowire diameters, and the oxide state of the outer layers of Si nanowires. The relationships between the geometry (size and spacing) of Si nanospheres, the initial state (diameter and oxide state) of Si nanowires, and the experimental conditions are discussed.
Microfour-point probe for studying electronic transport through surface states
Authors: Petersen C. L. , Grey F. , Shiraki I. , Hasegawa S.
Published: July 2002

Microfour-point probes integrated on silicon chips have been fabricated with probe spacings in the range 4–60 μm. They provide a simple robust device for electrical transport measurements at surfaces, bridging the gap between conventional macroscopic four-point probes and scanning tunneling microscopy. Measurements on Si(111) surfaces in ultrahigh vacuum reveal that the Si(111)–√3×√3–Ag structure induced by a monolayer of Ag atoms has a four-point resistance two orders of magnitude lower than that of the Si(111)–7×7 clean surface. We attribute this remarkable difference to direct transport through surface states, which is not observed on the macroscopic scale, presumably due to scattering at atomic steps.
Quantum study of the adsorption of small molecules on ice: The infrared frequency of the surface hydroxyl group and the vibrational stark effect
Authors: Manca C. , Allouche A.
Published: July 2002

A periodic Hartree–Fock quantum calculation has been developed for the adsorption of small nonpolar model systems (C2H2, C2H4, O3, CO, N2, and Ar) on ice in order to determine the physical variables responsible for the large infrared frequency shifts of the surface hydroxyls. It is shown that the correlation between these shifts and the corresponding interaction energies is not quite convincing. On the contrary it appears that the frequency shifts are tightly correlated to the variation with adsorption of the local electric field along the OH bond thus demonstrating that the dangling hydroxyl frequency is largely governed by the vibrational stark effect.
Relaxation mechanism of ferroelectric switching in Pb(Zr,Ti)O3 thin films
Authors: Lohse O. , Grossmann M. , Boettger U. , Bolten D. , Waser R.
Published: July 2002

The polarization reversal process of tetragonal Pb(Zr,Ti)O3 thin films has been intensively studied using conventional hysteresis and rectangle pulse measurements. Decreasing the voltage level of the pulses significantly slows down the polarization switching to the range of milliseconds. The switching current response shows a Curie–von Schweidler behavior over a broad time range. The transient current and the frequency dependence of the P–V loops of these films compared to the properties of ferroelectric single crystals show some similarities but also significant differences. The theoretical models of the classical ferroelectric phase transition and especially the conditions of the pulse measurements in single crystals and thin films are discussed. It leads to the conclusion that it is not the domain wall structure and domain wall motion that determine the polarization reversal but dissipative polarization processes which can take place in both ferroelectric and nonferroelectric high-k dielectric thin films.
Electronic continua in time-resolved photoelectron spectroscopy. I. Complementary ionization correlations
Authors: Blanchet V. , Zgierski M. Z. , Stolow Albert
Published: July 2002

We examine the role of electronic continua in time-resolved photoelectron spectroscopy studies of polyatomic nonadiabatic dynamics. We have investigated the two limiting cases for such studies. We consider here the limiting case of complementary ionization correlations where the two nonadiabatically coupled excited electronic states (S2 and S1) correlate (in the Koopmans’ picture) to different cation electronic states. We show, using an example of ultrafast internal conversion a linear polyene, that this favorable case allows for disentangling of the electronic population dynamics from the coupled vibrational dynamics. In the following paper, we investigate the unfavorable case of corresponding ionization correlations.
Interplay of non-Markovian relaxation and ultrafast optical state preparation in molecular systems: The Laguerre polynomial method
Authors: Mančal T. , May V.
Published: July 2002

The interplay of femtosecond optical excitation and retarded vibrational relaxation in a molecular system is studied using the non-Markovian version of the Quantum Master Equation. To solve non-Markovian equations with an arbitrary memory kernel an expansion with respect to Laguerre polynomials is introduced and the applicability of the method is tested. The non-Markovian effects are identified and parameter regimes are indicated where these effects become predominant. For an early time region just after the optical excited state preparation it is demonstrated that the convolutionless quantum master equation with a time-dependent Redfield-tensor may give a reasonable approximation of the correct non-Markovian dynamics.
Photoreflectance studies of surface state density of InAlAs
Authors: Hwang J. S. , Chang G. S. , Hwang W. C. , Chen W. J.
Published: July 2002

The surface barrier height and surface Fermi level of InAlAs were investigated via photoreflectance spectra. Surface state density was then determined from the surface barrier height as a function of temperature, illumination power intensity, and intrinsic layer thickness. Results obtained from these three independent approaches all give the same conclusion, that the surface states are distributed over two separate regions within the energy band gap. Closely examining the photovoltage induced by various incident beam intensities revealed that the photovoltage effect is negligible when the illumination power intensity is below 1.0 μW/cm2.
Growth and characterization of low-temperature grown GaN with high Fe doping
Authors: Akinaga H. , Németh S. , De Boeck J. , Nistor L. , Bender H. , Borghs G. , Ofuchi H. , Oshima M.
Published: July 2002

We succeeded in growing highly Fe-doped GaN films by solid-source molecular beam epitaxy using an electron-cyclotron-resonance microwave nitrogen plasma. The substrate temperature was in the range of 380–520 °C. The samples were analyzed by x-ray diffraction and transmission electron microscopy, and showed hexagonal (wurtzite) or cubic (zincblende) structure or a mixture of both phases. The Fe concentration was on the order of 1019 cm−3 and extended x-ray absorption fine structure data show that the Fe is substituting the Ga in GaN. The magnetization measurements as a function of temperature reveal ferromagnetic properties below 100 K for the sample grown at the lowest temperature.
Theoretical study of the water activation by a cobalt cation: Ab initio multireference theory versus density functional theory
Authors: Nakao Yoshihide , Hirao Kimihiko , Taketsugu Tetsuya
Published: July 2002

The reaction mechanism of Co+ (5F,3F) with H2O has been studied by the ab initio multireference-based theory (MR–SDCI and MC–QDPT) and the density functional theory (B3LYP and BLYP). In the energetics derived by the MR–SDCI(+Q) plus the B3LYP zero-point vibrational energy, the ion–dipole complex, CoOH2+, is initially formed with the binding energy of 38.2 (triplet) and 34.1 (quintet) kcal/mol, which is the most stable complex in the respective potential energy surfaces. Then, Co+ activates one O–H bond of H2O, leading to the insertion complex, HCoOH+. There are three possible dissociation channels from HCoOH+, i.e., →CoOH++H, →CoH++OH, and →CoO++H2. The third dissociation is expected to occur through the transition state of a four-centered structure, with the activation barrier of 61.6 (triplet) and 49.2 (quintet) kcal/mol, although this dissociation has not been detected in the experiment. The ground state of CoO+ is predicted to be Δ,5 and the lowest triplet state is Γ3 with the energy level of 20.8 kcal/mol above. The B3LYP provides the energetics qualitatively similar to the MR–SDCI(+Q) ones through the reactions, with the maximum deviation of 13 kcal/mol. The calculated results are consistent with experimental observations.
On the surface condition of Langmuir probes in reactive plasmas
Authors: Stamate E. , Ohe K.
Published: July 2002

The edge effect of a planar probe induces an elliptic-like sheath structure that acts as an electrostatic lens, which then focuses the charged particles on distinct regions of the probe surface. Positive-ion sputtering, chemical adsorption, and/or plasma deposition divide the probe surface into distinct regions with different work functions, which cause a double-hump structure (DHS) in the second derivative of the probe current. Thus, the DHS cannot be correlated with a distinct group of charged particles.
Thermal microcrack distribution control in GaN layers on Si substrates by lateral confined epitaxy
Authors: Zamir S. , Meyler B. , Salzman J.
Published: July 2002

GaN epitaxial layers grown uniformly on Si substrates suffer from randomly distributed thermal cracks. The growth on prepatterned Si substrates is demonstrated as an efficient way to control the geometrical distribution of the thermal cracks. In order to study this effect and to find the maximum crack-free lateral dimension of a GaN patterned unit on Si, a simple procedure termed lateral confined epitaxy (LCE) was developed. This procedure confines the growth of GaN to separate mesas of Si, which are defined on the Si substrate prior to the growth. The growth is performed by a single, continuous metalorganic chemical vapor deposition run. LCE enables the variation of mesa lateral size, while keeping the growth rate nearly unchanged. By performing a set of LCE growth runs of ∼0.7 μm GaN, on Si mesas of varying lateral dimensions, we specified the maximum crack-free range of GaN on Si as 14.0±0.3 μm, for that GaN thickness. A reduction of random crack density is observed with decreasing GaN pattern size and is explained in terms of shear-lag stress distribution.
The structure of Ni46, Ni47, and Ni48
Authors: Parks E. K. , Kerns K. P. , Riley S. J.
Published: July 2002

The geometrical structures of Ni46, Ni47, and Ni48 are investigated by the chemical probe method using molecular nitrogen as the probe molecule. Nitrogen saturation levels are determined and found to be consistent with cluster structures derived from two geometries, one having face-centered-cubic (fcc) packing and the other fcc/hcp (hexagonal-close-packed) layered packing. The fcc-based structures provide the best overall agreement with the experimental measurements. The saturation levels are inconsistent with icosahedral or decahedral packing, a structure found for Ni49 and larger clusters in earlier experiments. The change in cluster structure between Ni48 and Ni49 (most likely from fcc to icosahedral packing) is clearly evident in the nitrogen adsorption experiments at both low and high coverages, as well as with other reagent molecules.
Theory and numerical study of exciton dynamics in a disordered linear chain
Authors: Shimizu Makoto , Suto Shozo , Goto Takenari
Published: July 2002

We have formulated the exciton dynamics in a disordered linear chain with exciton wave functions given by the one-dimensional Frenkel exciton Hamiltonian with disorder. It is assumed that exciton–phonon coupling is weak and that the dynamics is governed by the competing processes of phonon scattering and radiative decay. The phonon scattering rate is given on the assumption that excitons do not change the site by the scattering. The strength of exciton–phonon coupling and the density of phonon states are independent of energy. The radiative decay rate is given by the Einstein’s A coefficient. The detail of the numerical procedure is also described. Absorption spectra, luminescence spectra, the time response of luminescence intensity, and temperature dependence are calculated for the model system of poly(di-n-hexylsilane) film. It is discussed that long-range dipole–dipole interaction is responsible for the luminescence depolarization.
Coherent optical control over collective vibrations traveling at lightlike speeds
Authors: Koehl Richard M. , Nelson Keith A.
Published: July 2002

Coherent optical control in both spatial and temporal dimensions is demonstrated. Lattice waves are generated by femtosecond pulses at one region of a crystal, monitored through spatiotemporal imaging which captures their propagation at lightlike speeds, and manipulated by additional pulses at other regions of the crystal distinct from the first. The capabilities demonstrated here enable terahertz polaritonics through enhanced optical control over propagating lattice excitations and ultrahigh-bandwidth, terahertz-frequency signals.
Self-assembled Fe nanowires using organometallic chemical vapor deposition and CaF2 masks on stepped Si(111)
Authors: Lin J.-L. , Petrovykh D. Y. , Kirakosian A. , Rauscher H. , Himpsel F. J. , Dowben P. A.
Published: July 2002

Linear arrays of 3 nm wide Fe stripes with 15 nm spacing are fabricated by self-assembly. They are formed by photolysis of ferrocene that is selectively adsorbed between CaF2 stripes. An ultraviolet nitrogen laser removes the organic ligands from ferrocene. Arrays of CaF2 stripes serve as masks, which are self-assembled on a stepped Si(111) surface. Scanning tunneling microscopy is used to investigate the surface morphology during growth. A generalization of this method to other wire materials is discussed.
Conformational dynamics in polyethylene under isochoric conditions: A molecular dynamics simulation study
Authors: Bharadwaj Rishikesh K. , Boyd Richard H.
Published: July 2002

Conformational dynamics in polyethylene (PE) have been studied via molecular dynamics simulations under isochoric conditions at four specific volumes (1.26, 1.20, 1.15 and 1.10 cm3 g−1) as a function of temperature (300–500 K). The autocorrelation function (ACF) decay for virtual perpendicular dipolar relaxation and rates of conformational transitions were monitored. The isochoric Kohlrausch–Williams–Watts (KWW) relaxation times determined from the ACFs show Vogel–Fulcher (V-F) behavior with temperature. From the pressures determined at each T, V point, constant pressure relaxation times vs temperature could also be constructed along several isobars by interpolation. The resulting temperature dependent activation energies at constant volume and at constant pressure are compared. At temperatures where the isochoric and isobaric P, V values coincide, i.e., where the ischoric and isobaric V-F curves intersect, the ratios of the constant volume activation energies to those at constant pressure are found to be ∼0.8. This high ratio can be considered as an indicator of activated barrier crossing dynamics dominating in comparison to the role of free volume. The dipolar ACF relaxation times diverge from the conformational transition rates as the temperature is lowered (under isochoric conditions) and analogously with increasing pressure (under isothermal conditions). The spatial heterogeneity of the conformational transitions increases with decreasing temperature and an increase in self-correlation of the conformational transitions ensues. These signatures of approaching vitrification are consistent with those previously found for PE under isobaric conditions. Qualitatively, decreasing the temperature at constant volume, increasing the pressure at constant temperature, and decreasing the temperature at constant pressure result in very similar behavior in the conformational dynamics.
Strain effects and atomic arrangements of 60° and 90° dislocations near the ZnTe/GaAs heterointerface
Authors: Kim T. W. , Lee D. U. , Lee H. S. , Lee J. Y. , Park H. L.
Published: July 2002

Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) measurements were carried out to characterize the composition of ZnTe films, and transmission electron microscopy (TEM) measurements were performed to investigate the lattice mismatch and the microstructural properties of the ZnTe/GaAs heterostructures. The AES and SIMS results showed that the ZnTe/GaAs heterointerfaces had relatively sharp interfaces. The TEM images and the selected-area electron-diffraction patterns showed a large lattice mismatch between the ZnTe epitaxial layer and the GaAs substrate, 60° and 90° dislocations together with stacking faults, near the ZnTe/GaAs heterointerface. The ZnTe epitaxial film grown on the GaAs substrate receives a compressive strain of −0.61%, and possible atomic arrangements of the 60° and the 90° dislocations are presented on the basis of the high-resolution TEM results.
Role of electronic excitations in explosive decomposition of solids
Authors: Kuklja Maija M. , Aduev B. P. , Aluker E. D. , Krasheninin V. I. , Krechetov A. G. , Mitrofanov A. Yu.
Published: July 2002

A combined theoretical and experimental study is performed for the initiation of chemistry process in high explosive crystals from a solid-state physics viewpoint. In particular, we were looking for the relationship between the defect-induced deformation of the electronic structure of solids, electronic excitations, and chemical reactions under shock conditions. Band structure calculations by means of the Hartree–Fock method with correlation corrections were done to model an effect of a strong compression induced by a shock/impact wave on the crystals with and without edge dislocations. Based on the obtained results, an excitonic mechanism of the earliest stages for initiation of high explosive solids is suggested with application to cyclotrimethylene trinitramine (also known as RDX) crystal. Experimental tests of this mechanism for AgN3 decomposition controlled by the dislocation density were worked out. The use of pulse radiolysis techniques allows us to observe pre-explosion modifications in properties and behavior of the solids. Pre-explosion conductivity and pre-explosion luminescence measurements for a series of heavy metal azides lead us to the model for the development of the decomposition chain reaction. Thus, the key role of electronic excitations facilitated by edge dislocations in explosive solids is established and analyzed. Practical applications of the suggested mechanisms are discussed.
Multicolor oligothiophene-based light-emitting diodes
Authors: Gigli G. , Inganäs O. , Anni M. , De Vittorio M. , Cingolani R. , Barbarella G. , Favaretto L.
Published: July 2002

We demonstrate wide tunability, from green to near infrared, of the electroluminescence emission of substituted oligothiophene compounds. The compounds are characterized by high chemical stability, electron affinities up to 3.1 eV and photoluminescence efficiencies up to 70%. These characteristics make these materials excellent candidates for application in light-emitting diodes. We obtain low turn-on voltage devices with electroluminescence efficiency up to 0.2%, more than one order of magnitude larger than the values reported for unsubstituted oligothiophene compounds.
Determination of an ethane intermolecular potential model for use in molecular simulations from ab initio calculations
Authors: Rowley Richard L. , Yang Yan , Pakkanen Tapani A.
Published: July 2002

Counterpoise-corrected, supermolecule, ab initio energies obtained at the MP2/6-311+G(2df,2pd) level were computed for 22 different relative orientations of two ethane molecules as a function of the separation distance between the molecular centers. These energies were used to regress the parameters in several simple, analytical, interatomic or site–site models that can be used for implementation in molecular simulations. Sensitivity analysis indicates that the intermolecular potential surface is insensitive to C–C interactions and that the parameters in the C–C model are coupled and unobtainable from the dimer energies. Representation of the potential surface can be made in terms of C–H and H–H interatomic potentials if the C–C interactions are treated as shielded. Simple Lennard-Jones and exp-6 models do not adequately represent the potential surface using these shielded models, nor do they produce the anticipated physics for the interatomic potentials. The exp-6 model with a damping function and the modified-Morse interatomic potentials both reproduce the intermolecular potential surface well with physically realistic intersite potentials suitable for use in molecular dynamics simulations.
The ab initio model potential method with the spin-free relativistic scheme by eliminating small components Hamiltonian
Authors: Motegi Kyosuke , Nakajima Takahito , Hirao Kimihiko , Seijo Luis
Published: July 2002

A relativistic ab initio model potential (AIMP) for Pt, Au, and Hg atoms has been developed using a relativistic scheme by eliminating small components (RESC) in which the 5p, 5d, and 6s electrons are treated explicitly. The quality of new RESC–AIMP has been tested by calculating the spectroscopic properties of the hydrides of these elements using the Hartree–Fock and coupled cluster with singles and doubles (CCSD) methods. The agreement with reference all-electron RESC calculations is excellent. The RESC–AIMP method is applied successfully in the investigation of the spectroscopic constants of Au2 and Hg2 using the CCSD method with a perturbative estimate of the contributions of triples. The ground state of Pt2 is also determined by RESC–AIMP with the second-order complete active space perturbation method. The results show that scalar relativistic effects on the valence properties are well described by the RESC–AIMP method. The effect on the basis set superposition error on the spectroscopic constants is also examined.
Stimulated Brillouin and Raman scattering from a randomized laser beam in large inhomogeneous collisional plasmas. II. Model description and comparison with experiments
Authors: Tikhonchuk V. T. , Fuchs J. , Labaune C. , Depierreux S. , Hüller S. , Myatt J. , Baldis H. A.
Published: July 2002

A model for stimulated Brillouin (SBS) and Raman (SRS) backscattering of a spatially smoothed laser beam interacting with a collisional, inhomogeneous, expanding plasma is presented. It is based on the independent hot spots description [H. A. Rose and D. F. DuBois, Phys. Rev. Lett. 72, 2883 (1994)], in which the overall plasma reflectivity is assumed to be a sum of the individual speckle reflectivities. Self-focusing is taken into account in the computation of the speckle intensity profile and reflectivities. Two additions have been made to previous similar theories: (i) the thermal effects are retained along with the ponderomotive force for what concerns speckle self-focusing, and (ii) SRS (convective and absolute) is accounted for in calculations of the speckle reflectivity. The model is benchmarked against recent laser–plasma experiments at Laboratoire pour l’Utilisation des Lasers Intenses, at École Polytechnique, France, with well-characterized interaction conditions. A good agreement is found between the experimental SBS levels and the model calculations using the measured plasma parameters. This agreement applies for two types of beam smoothing techniques, random phase plates, and polarization smoothing, various plasma densities, and laser energies. Self-focusing itself, and thermal effects in it, play both a fundamental role in defining the level of plasma backscattering. The absolute Raman instability in speckles dominates the SRS response. The model predictions for the SRS reflectivity are less satisfactory, although they demonstrate the same trends as the experimental data. It follows from model calculations and experimental data that the polarization smoothing technique provides an efficient method of control of parametric instabilities allowing a reduction of several times in the level of SBS and SRS reflectivities.
From ferromagnetic–ferromagnetic to ferromagnetic–antiferromagnetic exchange coupling in NiFe/MnNi bilayers
Authors: Spenato David , Youssef Jamal Ben , Le Gall Henri , Ostoréro Jean
Published: July 2002

The effect of the growth conditions and the Mn concentration on the exchange coupling between a ferromagnetic (F) NiFe and an antiferromagnetic (AF) MnNi layers were studied. We found that an F/AF coupling appears in the bilayers when the Mn concentration is more than 45%. Beyond this critical concentration the exchange field shows a maximum then decreases. The correlation between the exchange field and the microstructure of the film is discussed. We show that: (1) the enhancement of the exchange field is associated with the enhancement of the antiferromagnetic grain size and (2) the existence of the exchange field is associated with a third x-ray peak which may be an FeMnNi ternary allow type. This result was associated with interfacial diffusion confirmed by magnetization variation measurements before and after annealing.
Enhancement of giant magnetoresistance properties of NiO spin valves with an oxidized free layer naturally in the air
Authors: Kim Jong-Kee , Hwang Do-Guwn , Lee Sang-Suk , Joo Ho-Wan , Lee Ky-Am , Park Chang-Man
Published: July 2002

The effect of specular electron scattering on the natural oxidation of a free layer in NiO spin valves has been investigated. The magnetoresistance ratio and the exchange biasing field (Hex) of NiO(600 Å)/Ni81Fe19(50 Å)/Co(7 Å)/Cu(20 Å)/Ni81Fe19(70 Å) spin valves were increased from 4.9% to 7.3%, and 110 to 170 Oe after the natural oxidation in the atmosphere for 80 days, respectively. The sheet resistance ρ decreased from 28 to 17 μΩ m, but Δρ did almost not change after the oxidation. The spin valves enhanced by the specular electron scattering in the naturally oxidized Co/NiFe/NiFeOx free layer were confirmed from the depth profiles using Auger electron spectroscopy.
Strain effects in thin films of CrO2 on rutile and sapphire substrates
Authors: Stampe P. A. , Kennedy R. J. , Watts S. M. , von Molnár S.
Published: July 2002

Detailed x-ray diffraction characterizations were made of chromium dioxide films fabricated by chemical vapor deposition onto (100) and (110) oriented TiO2 and (0001) Al2O3 substrates. Pole figures were used to examine the epitaxy of these systems, and the lattice parameters were calculated using x-ray area maps. The film on (100) TiO2 exhibits the best epitaxy of the three; however, it is significantly strained relative to bulk CrO2. The film on (110) TiO2 is distorted from the ideal tetragonal structure by 0.17° in the angle between the a and b lattice directions, and also exhibits significant mosaicity. The film on sapphire contains crystallites that have grown in three in-plane orientations, but exhibit the best rocking curve widths and the least degree of strain of the films studied. Magnetization and magnetotransport measurements are shown to demonstrate effects of strain and crystalline structure on the physical properties of these films.
Modeling the response of turbulence subjected to cyclic irrotational strain
Authors: Hadžić I. , Hanjalić K. , Laurence D.
Published: July 2002

Using a second moment closure, analytical solutions for homogeneous turbulence subjected to periodic compression-dilatation strains show that both the characteristic turbulence frequency and turbulence kinetic energy eventually decay, irrespective of the initial turbulence level, anisotropy of the stress field, or Reynolds number. The eddy-viscosity models give erroneous results because of the artificial positive generation of turbulence energy during both the compression and expansion phase. The first observation results from the phase lag between periodic strain rate and stresses introduced by the exact production term in the second moment closure, whereas the eddy-viscosity model synchronizes the stresses with the strain rate, resulting in an overestimation of turbulence generation. The above findings are illustrated by analytical solutions, as well as by numerical solutions of in-cylinder turbulence, using the k−ε eddy-viscosity and the second-moment closure models. The analysis and simulations support the conjecture that turbulence submitted to cyclic strains should always finally decay.
Observation of current crowding near fabricated voids in gold lines
Authors: Yongsunthon R. , Stanishevsky A. , McCoy J. , Williams E. D.
Published: July 2002

The spatial variation of current density in lines with model void defects fabricated using focused-ion beam milling has been imaged using magnetic force microscopy (MFM). At current densities of 3–4×106 A/cm2, an asymmetry in the MFM signal is clearly visible at (1×1) μm2 and (0.5×0.5) μm2 notches at the edge of a 10 μm wide line. Comparison to a simple model calculation suggests that the asymmetry is due to current crowding, with the displaced current 70% localized to within 1μm of the notch.
A small disturbance in the strong vortex flow
Authors: Klimenko A. Y.
Published: July 2002

A small disturbance in the axisymmetric, bathtub-like flow with strong vorticity is considered and the asymptotic representation of the solution is found. It is shown that if the disturbance is smaller than a certain critical scale, the conventional strong vortex approximation cannot describe the field generated by the disturbance not only in the vicinity of the disturbance but also at the distances much larger than the critical scale.
Ionization instabilities of an electromagnetic wave propagating in a tenuous gas
Authors: Bian Zhigang , Antonsen Thomas M.
Published: July 2002

A theory is developed to study the scattering instability that occurs when a laser pulse propagates through and ionizes a gas. The instability is due to the intensity dependence of the ionization rate, which leads to a transversely structured free electron density. The instability is convective in the frame of the laser pulse, but can have a relatively short growth length scaling as Lg∼k0/kp2, where k0 is the laser wave number, kp2=ωp2/c2 and ωp is the plasma frequency. The most unstable perturbations correspond to a scattering angle for which the transverse wave number is around the plasma wave number, kp. The scattered light is frequency upshifted. The comparison between simple analytic theory and numerical simulation shows good agreement.
Experimental study of the plasma resonance in a planar surface wave plasma
Authors: Wu T. J. , Guan W. J. , Tsai C. M. , Yeh W. Y. , Kou C. S.
Published: July 2002

In this study, the plasma resonance in a surface wave sustained plasma was investigated. Surface wave discharges have been successfully generated by a microwave planar vane-type slow wave structure. Experimental results clearly demonstrated that the amplitude of the electric field of the microwave always became a local maximum in the position where the plasma density was equal to the same value. When the operating gas pressure and the input microwave power were varied to change the plasma density profile, the location of the maximum of the electric field was found to shift in compliance with the plasma density profile. Meanwhile, the peak of the resonance response was found to be proportional to the plasma density gradient while the spatial width of plasma resonance was inversely proportional to the plasma density gradient. These results were qualitatively in accordance with the theory.
Evolution of functional model proteins
Authors: Blackburne Benjamin P. , Hirst Jonathan D.
Published: July 2002

The distinct influences of function, folding, and structure on the evolution of minimalist model proteins are studied by characterization of their evolutionary landscapes. Chains of up to 23 monomers on a two-dimensional square lattice are investigated by exhaustive enumeration of conformation and sequence space. In addition to common aspects of minimalist models, such as unique, stable native states and cooperative folding, functional model proteins have the novel feature of an explicit binding pocket. Fitness is defined through simple, physical characterization of the binding pocket. We characterize various properties of functional model proteins, focusing on their evolutionary landscapes, as defined by single point mutations, insertions, and deletions. The longer chains more closely resemble real proteins, having richer functional diversity and forming larger families of sequences. Although regions of evolutionary landscapes are often highly interconnected, we also observe so-called critical pathways, where evolution can only proceed through a single set of mutants.
Strontium clusters: Many-body potential, energetics, and structural transitions
Authors: Wang Guan Ming , Blaisten-Barojas Estela , Roitberg A. E. , Martin T. P.
Published: July 2002

A many-body potential for strontium clusters is developed with parameters fitted to the energy surface of strontium clusters containing up to ten atoms calculated within the density functional theory in the generalized gradient approximation. Structure and energetics of the most stable cluster isomers with up to 63 atoms are obtained with genetic algorithms. Additionally, the high resolution mass spectrum of strontium clusters up to Sr96 at finite temperature is provided. Several thermodynamic properties are studied under the many-body potential as a function of temperature. It is found that stability patterns, indicating how stable a cluster size is with respect to its neighboring sizes, change significantly with temperature. This behavior is due to structural transitions of the strontium clusters that occur at finite temperatures. A comparison with the experimental mass abundance indicates that only the structures above 400 K were observed experimentally. Very prominent magic numbers are predicted at 34 and 61.
Statistical mechanics of solvophobic aggregation: Additive and cooperative effects
Authors: Shimizu Seishi , Chan Hue Sun
Published: July 2002

Effects of possible non-pairwise-additive interactions on solvophobic aggregation are analyzed. A simple lattice model of binary solution with attractive solute-solute interactions is introduced to delineate the role of multiple-body effects in solute clustering and aggregation. Additive (noncooperative), cooperative, and anti-cooperative intersolute interactions are modeled by multiple-solute potentials that are respectively equal to, more favorable than, and less favorable than the sum of pairwise solute interactions. Under appropriate conditions, pairwise additive interactions and even interactions with significant anti-cooperativity can lead to aggregation and demixing. Cooperative interactions are not necessary for solute aggregation. Similarities and differences between solute aggregation and hydrophobic collapse of proteinlike heteropolymers are investigated. On average, heteropolymer collapse transitions as a function of solvophobic composition are significantly less sharp than the corresponding solute aggregation transitions. This difference is seen as a direct consequence of chain connectivity constraints.
Positron annihilation inside C60
Authors: Ito Yutaka , Iwasa Yoshihiro , Tuan Nguen Mahn , Moriyama Shin-ichi
Published: July 2002

It has been considered that positrons in pure C60 are distributed in the interstitial sites between the C60 molecules. However, it is possible to reside a positron inside a C60 molecule when the interstitial space between the C60 molecules become filled with alkali atoms. We report here the positron-annihilation characteristics, lifetime and Doppler-broadening for the annihilation radiation, in K6C60. We have observed that both the lifetime and Doppler-broadening spectrum of K6C60 can be separated into two components. In both measurements, the value of one component is close to the value observed for positron annihilation in pure C60, where a positron annihilates with an electron outside of a C60 molecule. On the other hand, we have observed for K6C60 the short lifetime (197 ps), and the extremely small Doppler-broadening on the annihilation spectrum corresponding to the mean electron momentum distribution of 0.95×10−3 m0c [full width at half maximum (FWHM)]. These components are considered to reflect positron annihilation inside a C60 molecule. These demonstrations will lead to further applications to spectroscopy inside a C60 molecule.
Energy dissipation and scattering angle distribution analysis of the classical trajectory calculations of methane scattering from a Ni(111) surface
Authors: Milot Robin , Kleyn A. W. , Jansen A. P. J.
Published: July 2002

We present classical trajectory calculations of the rotational vibrational scattering of a nonrigid methane molecule from a Ni(111) surface. Energy dissipation and scattering angles have been studied as a function of the translational kinetic energy, the incidence angle, the (rotational) nozzle temperature, and the surface temperature. Scattering angles are somewhat toward the surface for the incidence angles of 30°, 45°, and 60° at a translational energy of 96 kJ/mol. Energy loss is primarily from the normal component of the translational energy. It is transferred for somewhat more than half to the surface and the rest is transferred mostly to rotational motion. The spread in the change of translational energy has a basis in the spread of the transfer to rotational energy, and can be enhanced by raising of the surface temperature through the transfer process to the surface motion.
Oxygen exchange kinetics on a highly oriented La0.5Sr0.5CoO3−δ thin film prepared by pulsed-laser deposition
Authors: Yang Yuemei L. , Jacobson A. J. , Chen C. L. , Luo G. P. , Ross K. D. , Chu C. W.
Published: July 2002

Oxygen exchange at a highly oriented La0.5Sr0.5CoO3−δ thin film prepared on (100) surfaces of an yttria-stabilized zirconia single crystal by pulsed-laser deposition was studied with ac impedance spectroscopy under various temperatures and oxygen partial pressures. Three distinctive features observed in the impedance spectra were assigned to contributions from the ionic conduction of the electrolyte, oxide ion transfer across the electrode/electrolyte interface, and the oxygen exchange on the film surface. An equivalent circuit model was proposed to analyze the impedance results, from which the surface chemical exchange coefficients, kchem, were derived as a function of temperature and oxygen partial pressure.
Ab initio study of the lower few states of FeH: Application of the multireference coupled pair approximation
Authors: Tanaka Kiyoshi , Sekiya Masahiro , Yoshimine Megumu
Published: July 2002

Multireference coupled pair approximation(4) [MRCPA(4)] was applied to describe the ground state and the lower excited states of FeH. This study demonstrates that the a 6Δ state is 0.27 eV above the ground state, X 4Δ, which is in good agreement with the observation (0.25 eV). The ground state is much more highly correlated than the a 6Δ state and the use of the size-consistent method is important to predict the relative stability accurately. In addition to the above results, spectroscopic data of the second Δ,4 the lowest Π,4 the lowest Φ4, the lowest two Π6, and the lowest Σ+6 states are reported. The calculated excitation energies of the lowest Σ+6 state and the second Δ4 state are in good agreement with results of experiment. The total energies of the lowest Δ4, Π,4, Δ6, Π6 and Σ+6 states are in the order of Δ4<Π4<Δ6<Π6<Σ+6 which supports what was anticipated previously.
An integral equation theory for polymer solutions: Explicit inclusion of the solvent molecules
Authors: Mendez Sergio , Curro John G. , Pütz Mathias , Bedrov Dmitry , Smith Grant D.
Published: July 2002

Self-consistent Polymer Reference Interaction Site Model (PRISM) calculations and molecular dynamics (MD) simulations were performed on athermal solutions of linear polymers. Unlike most previous treatments of polymer solutions, we explicitly included the solvent molecules. The polymers were modeled as tangent site chains and the solvent molecules were taken to be spherical sites having the same intermolecular potential as the polymer sites. The PRISM theory was solved self-consistently for both the single chain structure and intermolecular correlations as a function of chain length and concentration. The rms end-to-end distance from PRISM theory was found to be in agreement with corresponding MD simulations, and exhibited molecular weight dependence in accordance with scaling predictions in the dilute and concentrated solution limits. The presence of explicit solvent molecules had a significant effect on the packing of the polymer by inducing additional structure in the intermolecular radial distribution function between polymer sites. Using the direct correlation functions from the athermal solution and the random phase approximation, we were able to estimate the spinodal curves for solutions when polymer and solvent attractions were turned on. We found significant deviations from Flory–Huggins theory that are likely due to compressibility and nonrandom mixing effects.
Test of variational transition state theory with multidimensional tunneling contributions against an accurate full-dimensional rate constant calculation for a six-atom system
Authors: Pu Jingzhi , Corchado José C. , Truhlar Donald G.
Published: July 2002

We present calculations of the H+CH4 reaction rate on the Jordan–Gilbert surface using canonical variational transition state theory with microcanonical optimized multidimensional tunneling contributions (CVT/μOMT). The purpose of the calculation is to compare the results to the recent accurate dynamical calculations of Bowman, Wang, Huang, Huarte-Larrañaga, and Manthe for this potential energy surface. Over the full 200–500 K range for which accurate results are available we find a mean absolute deviation of only 17% and a maximum absolute deviation of 23%. This provides a rigorous validation of this popular method for a larger system than has previously been possible and indicates that previous validations for atom–diatom reactions were indeed indicative of the kind of accuracy one can obtain for larger systems.
Spin engineering in ultrathin Co0.35Pd0.65 alloy films
Authors: Kim Sang-Koog , Lee Jeong-Won , Jeong Jong-Ryul , Kim Jonggeol , Shin Sung-Chul
Published: July 2002

The easy axis of magnetization in CoxPd1−x alloy films with x=0.35 is controllably engineered by varying the thickness, tPd, of the Pd overlayers directly deposited on the alloy layers. In a Pd(50 Å)/CoPd (20 Å)/Pd (tPd) sample with a 10-Å-height step-wedge Pd layer, the easy axis smoothly changes from in-plane orientation (tPd=0 Å) through canted out of plane (0<tPd<30 Å) to perpendicular (30⩽tPd⩽60 Å). We also demonstrate that the spin switching is controllably reversible between in-plane and perpendicular orientations when the individual constituent layers of CoPd and Pd are alternately deposited. Smoothly continuous spin reorientation in a Pd (50 Å)/CoPd (30 Å)/Pd (tPd) film with increasing tPd in a broad range of 0–150 Å convincingly evidences the magnetoelastic anisotropy origin for the observed spin switching.
Further developments in an ac-based capacitance tomography system
Authors: Yang W. Q.
Published: July 2002

An ac-based electrical capacitance tomography (ECT) system has been developed at UMIST, with sinusoidal excitation and phase-sensitive demodulation (PSD). In this article the overall system design is briefly described and then discussed in detail are the key design features, needed to achieve a high signal-to-noise ratio and a high data acquisition rate, including transient process analysis of the first stage, design of an analog Butterworth low-pass filter and optimization of the data acquisition process. The experiments show that the ac-based system has signal variance 19 times better than the previous design, the charge/discharge ECT system, and that a data acquisition rate of 140 frames per second is achieved for a 12-electrode system.
Note on the thermal stresses in passivated metal interconnects
Authors: Sharma P. , Ardebili H. , Loman J.
Published: July 2002

An analytical model to compute thermal stresses in passivated metal interconnects is proposed in this article. Typical aspect ratio of passivated metal interconnects is 1 or less (frequently between 0.5 and 1). Previous Eshelby-based analytical model by Niwa et al. [J. Appl. Phys. 68, 328 (1990)] is not very accurate as it fails to take into account the proximity of the interconnect to the free surface of the passivation. A recently proposed model by Wikström et al. [J. Appl. Phys. 86, 6088 (1999)] precisely does not work when the aspect ratio < 1. The analytical model proposed in this letter can predict stresses (average and spatial variations) in passivated metal interconnects with superior accuracy. The effect of free surface of passivation is fully taken into account and comparisons with previous works are presented.
Effects of interdiffusion on the band alignment of GeSi dots
Authors: Wan J. , Luo Y. H. , Jiang Z. M. , Jin G. , Liu J. L. , Wang Kang L. , Liao X. Z. , Zou J.
Published: July 2002

The interdiffusion effects on the band alignment of the GeSi dots embedded in Si matrix were studied by temperature- and excitation-power-dependent photoluminescence measurements. A different power-dependent behavior of the photoluminescence for the as-grown and the annealed samples was observed. It was suggested that the band alignments of the dots changed from type II to type I after annealing due to the Ge/Si interdiffusion. The decrease of the valence band offset, which was also induced by the Ge/Si interdiffusion, was observed from the temperature-dependent photoluminescence measurements.