bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of the D line in the Raman spectrum of graphite: A study based on Raman frequencies and intensities of polycyclic aromatic hydrocarbon molecules
Authors: Castiglioni C. , Mapelli C. , Negri F. , Zerbi G.
Published: July 2002

A common valence force field for polycyclic aromatic hydrocarbons (PAHs) and graphite has been used in order to explain their Raman spectra from a unified viewpoint. On this basis the correlation observed between experimental spectra has been explained and rationalized. Quantum chemical density functional theory calculations of Raman intensities of small PAHs have been also performed, supporting the conclusions obtained from the dynamical analysis. The results obtained are useful for the characterization of materials containing graphitic domains and provide some new insight on the nature of the D peak in disordered graphite.
Valence band spectra of nitrogen incorporated amorphous carbon films
Authors: Bhattacharyya Somnath , Spaeth C. , Richter F.
Published: July 2002

Tetrahedral amorphous carbon (ta-C) films were deposited in a filtered cathodic vacuum arc chamber. Nitrogen, of atomic concentration up to 30%, was introduced in the films during deposition by a Kaufmann-ion source. Change of the film structure and the valence band (VB) spectra of ta-C film due to nitrogen incorporation was studied by ultraviolet photoelectron spectroscopy (UPS) using He I and He II excitations as well as x-ray photoelectron spectroscopy (XPS). A comparative study of the electronic structure between ta-C and the nitrogenated films was demonstrated by decomposition of their VB spectra into several bands and from the intensity difference of these spectra. An additional density of states close to the Fermi level (EF), representing the nitrogen lone pair state, has been detected from both UPS and XPS VB spectra of nitrogenated samples. From the shift of the VB relative to the EF nitrogen doping of ta-C is demonstrated. The change of the density of states at the edge of VB and especially the C 2s and N 2s states is thoroughly explained. The modification of the structure of nitrogenated films prepared by applying the substrate bias and temperature was also studied through comparison of the VB spectra.
Self-assembling three-dimensional colloidal photonic crystal structure with high crystalline quality
Authors: Ye Yong-Hong , LeBlanc François , Haché Alain , Truong Vo-Van
Published: July 2002

High-quality colloidal crystal multilayers were fabricated from aqueous solutions by the vertical deposition method. The effect of the evaporation temperature on the crystalline quality of colloidal crystals was carried out. It is found that with the increase of the evaporation temperature, the colloidal crystal shows an increasing tendency towards equilibrium face-centered-cubic phase, and the resulted sample also shows few dislocations and vacancies when the balance in the processes of nucleus formation, particle transport, and crystallization can be kept. However, with the further increase of the evaporation temperature (above 55 °C), a vast amount of defects appear in the crystal because the fast water evaporation rate, which results in a fast crystal growth rate, will spoil the balance. Optical measurements correspond well to the microstructure results.
Deformation of surfaces, integrable systems, and Chern–Simons theory
Authors: Martina L. , Myrzakul Kur. , Myrzakulov R. , Soliani G.
Published: July 2002

A few years ago, some of us devised a method to obtain integrable systems in (2+1)-dimensions from the classical non-Abelian pure Chern–Simons action via the reduction of the gauge connection in Hermitian symmetric spaces. In this article we show that the methods developed in studying classical non-Abelian pure Chern–Simons actions can be naturally implemented by means of a geometrical interpretation of such systems. The Chern–Simons equation of motion turns out to be related to time evolving two-dimensional surfaces in such a way that these deformations are both locally compatible with the Gauss–Mainardi–Codazzi equations and completely integrable. The properties of these relationships are investigated together with the most relevant consequences. Explicit examples of integrable surface deformations are displayed and discussed.
Characteristic enhancement of emission from SrTiO3:Pr3+ by addition of group-IIIb ions
Authors: Okamoto Shinji , Yamamoto Hajime
Published: July 2002

Enhancement of the emission from SrTiO3:Pr3+ by addition of group-IIIb ions (IIIb3+) has been investigated. From the photoluminescence (PL) spectra and x-ray diffraction patterns, it is speculated that one of the origins of the enhancement is charge compensation of Pr3+ at the Sr2+ site by IIIb3+ substituting for Ti4+ around Pr3+, which results in a change in the crystal field around Pr3+. The IIIb3+-dependent PL spectra can be explained qualitatively by a local distortion around Pr3+ ions due to a difference in an ionic radius between Ti4+ and IIIb3+.
The recoiling of liquid droplets upon collision with solid surfaces
Authors: Kim H.-Y. , Chun J.-H.
Published: July 2002

Although the spreading behavior of liquid droplets impacting on solid surfaces has been extensively studied, the mechanism of recoiling which takes place after the droplet reaches its maximum spread diameter has not yet been fully understood. This paper reports the study of the recoiling behavior of different liquid droplets (water, ink, and silicone oil) on different solid surfaces (polycarbonate and silicon oxide). The droplet dynamics are experimentally studied using a high speed video system. Analytical methods using the variational principle, which were originated by Kendall and Rohsenow (MIT Technical Report 85694-100, 1978) and Bechtel et al. [IBM J. Res. Dev. 25, 963 (1981)], are modified to account for wetting and viscous effects. In our model, an empirically determined dissipation factor is used to estimate the viscous friction. It is shown that the model closely predicts the experimental results obtained for the varying dynamic impact conditions and wetting characteristics. This study shows that droplets recoil fast and vigorously when the Ohnesorge number decreases or the Weber number increases. Droplets with a large equilibrium contact angle are also found to recoil faster. Here the Ohnesorge number scales the resisting force to the recoiling motion, and is shown to play the most important role in characterizing the recoiling motion.
Evolution of point defect clusters in pure iron under low-energy He+ irradiation
Authors: Arakawa K. , Imamura R. , Ohota K. , Ono K.
Published: July 2002

The formation process of point defect clusters in high-purity (99.999%) iron as a typical bcc metal under the irradiation with low-energy (5 keV)He+ is studied by in situ transmission electron microscopy (TEM). Using conventional TEM techniques, clusters induced by the irradiation are determined to be interstitial-type dislocation loops (I loops) at temperatures ranging from 85 to 770 K and cavities from 300 to 770 K. Most of the I loops are determined to lie on {100} planes, and their Burgers vector are determined to be a[100]. The temporal variation in the volume density of I loops is measured by stereomicroscopy at several temperatures. The volume density of I loops is about two orders of magnitude higher in comparison with the case of high-energy electron irradiation at the same dpa rate; hence, it is deduced that helium atoms have an effect enhancing the nucleation of I loops. The depth distribution of the loop densities becomes broad above around 235 K at which vacancies become thermally mobile, and the dependence of loop formation on He+ beam flux is weak. From these results, it is suggested that helium-vacancy complexes act as trapping sites of self-interstitial atoms and I-loop nucleation is enhanced.
Numerical simulation of pulsating turbulent channel flow
Authors: Scotti Alberto , Piomelli Ugo
Published: July 2002

Direct and large-eddy simulations of the Navier–Stokes equations are used to study the pulsating flow in a channel. The cases examined span a wide range of frequencies of the driving pressure gradient, and encompass different physical behaviors, from the quasi-Stokes flow observed at high frequencies, to a quasisteady behavior at the lowest ones. The validity of the dynamic Smagorinsky model to study this kind of unsteady flow is established by a posteriori comparison with direct simulations and experimental data. It is shown that the fluctuations generated in the near-wall region by the unsteady pressure gradient do not propagate beyond a certain distance lt from the wall, which can be estimated quite accurately by a simple eddy viscosity argument. No substantial departure from the Stokes regime at very high frequency (ω+ as high as 0.1) is observed. The time-dependent characteristics of the flow are examined in detail, as well as the topology of the coherent structures.
Microsolvation of the water cation in neon: Infrared spectra and potential energy surface of the H2O+–Ne open-shell ionic complex
Authors: Dopfer Otto , Roth Doris , Maier John P.
Published: July 2002

The intermolecular potential of the H2O+–Ne open-shell ionic dimer in its doublet electronic ground state has been investigated by infrared spectroscopy in the vicinity of the O–H stretch vibrations (ν1 and ν3) and ab initio calculations at the unrestricted Møller–Plesset second-order (MP2) level with a basis set of aug-cc-pVTZ quality. The rovibrational structure of the photodissociation spectrum is consistent with a proton-bound planar H–O–H–Ne structure and a Ne–H separation of R0=1.815(5) Å. The complexation-induced redshifts are Δν1=−69 cm−1 and Δν3=−6 cm−1, respectively. Tunneling splittings observed in the perpendicular component of the ν3 hybrid band of H2O+–Ne are attributed to hindered internal rotation between the two equivalent proton-bound equilibrium structures. The interpretation of the H2O+–Ne spectrum is supported by the spectrum of the monodeuterated species, for which both the proton-bound and the deuteron-bound isomers are observed (DOH+–Ne, HOD+–Ne). The equilibrium structure of the calculated potential energy surface of H2O+–Ne has a slightly translinear proton bond, which is characterized by a Ne–H separation of Re=1.77 Å, a bond angle of φe=174°, and dissociation energies of De=756 cm−1 and D0=476 cm−1. According to the calculated potential, the exchange tunneling between the two equivalent minima occurs via the planar bridged transition state with C2v symmetry and a barrier of 340 cm−1. In general, the calculated properties of H2O+–Ne show good agreement with the experimental data. Initial steps in the microsolvation of the water cation in neon are discussed by comparing the calculated and experimental properties of H2O+–Nen (n=0–2) with neon matrix isolation data (n→∞).
Fabrication of nanoperiodic surface structures by controlled etching of dislocations in bicrystals
Authors: Wind Rikard A. , Murtagh Martin J. , Mei Fang , Wang Yu , Hines Melissa A. , Sass Stephen L.
Published: July 2002

A method for the fabrication of periodic arrays of surface features with controlled spacings of 2–100 nm has been developed. This process relies on the selective etching of dislocations formed at a twist–bonded interface in a bicrystal. The production of nanoscale periodic silicon surface features with a mean spacing of 38 nm is reported. The etch rate of edge and screw dislocations is compared, and the rate of dislocation etching is found to be poorly correlated to strain. This observation calls long-held theories of dislocation etching into question.
Young’s modulus measurements and grain boundary sliding in free-standing thin metal films
Authors: Kalkman A. J. , Verbruggen A. H. , Janssen G. C. A. M.
Published: July 2002

Young’s modulus of free-standing polycrystalline Al, Au, and W films with submicron thickness has been studied using a dynamic bulge-testing technique. For Au and Al films a clear frequency dependence of the modulus is observed at room temperature in the range 1×10−4–0.5 rad/s. The values of the moduli are considerably smaller than the corresponding values of bulk material. The modulus of W films measured under the same conditions does not depend on frequency and is equal to the bulk value. The origin of the behavior found in the Al and Au films is anelastic grain boundary sliding. As a consequence of the relatively small grain size of thin polycrystalline films this phenomenon is observable at room temperature in films with a relatively low melting point.
Thickness and drainage of perfluoropolyethers under compression
Authors: Xu Lei , Ogletree D. Frank , Salmeron Miquel , Tang Huan , Ma Xiading , Gui Jing
Published: July 2002

The Surface Forces Apparatus was used to study the compression and drainage of perfluoropolyethers (PFPEs) between two flat parallel mica surfaces. In the case of Zdols and Demnum-SA, the PFPEs can be squeezed out during slow compression to a final residual film one gyration diameter in thickness. This thickness remained constant up to the highest applied pressure of ∼10 MPa. The residual thickness for Demnum-SA, with one active end group, was found to be approximately 40% larger than that for Zdol of the same molecular weight, with two active end groups. In contrast, Z03, with no active end groups, could be displaced completely from the contact. The dynamics of expulsion were studied by monitoring the variation of the gap width as a function of time after fast (a few milliseconds) step increase in the compressive load. It was found that Zdol behaves as the bulk liquid down to gap widths of four equivalent gyration diameters. A viscosity increase of more than ten times was observed when the gap width was between four and two gyration diameters. Finally, slow compression to the maximum achievable pressure (∼10 MPa) led to a residual layer one gyration diameter in thickness trapped between the mica surfaces.
Microscopic structure of hydrogen impurity in LiNbO3
Authors: Nahm H. H. , Park C. H.
Published: July 2002

We investigate the microscopic structures of interstitial and substitutional hydrogen impurities in LiNbO3 through the first-principles pseudopotential total-energy calculations. The interstitial hydrogen is located between two O atoms and bonds to one of the oxygen atoms. The hydrogen impurity substituting Li is significantly displaced from the Li-site and strongly bonds to an oxygen atom and is located on a biaxial axis of an O triangle. We discuss the effect of hydrogen to the ferroelectric polarization and the energetics for the trap of the interstitial hydrogen at a Li vacancy.
Superconducting MgB2 films via precursor postprocessing approach
Authors: Paranthaman M. , Cantoni C. , Zhai H. Y. , Christen H. M. , Aytug T. , Sathyamurthy S. , Specht E. D. , Thompson J. R. , Lowndes D. H. , Kerchner H. R. , Christen D. K.
Published: July 2002

Superconducting MgB2 films with Tc=38.6 K were prepared using a precursor-deposition, ex situ postprocessing approach. Precursor films of boron, ∼0.5 μm thick, were deposited onto Al2O3 (102) substrates by electron-beam evaporation; a postanneal at 890 °C in the presence of bulk MgB2 and Mg metal produced highly crystalline MgB2 films. X-ray diffraction indicated that the films exhibit some degree of c-axis alignment, but are randomly oriented in plane. Transport current measurements of the superconducting properties show high values of the critical current density and yield an irreversibility line that exceeds that determined by magnetic measurements on bulk polycrystalline materials.
Investigation of adsorption and absorption-induced stresses using microcantilever sensors
Authors: Hu Zhiyu , Thundat T. , Warmack R. J.
Published: July 2002

The interaction between a vapor and a thin film adsorbed on one side of a bimaterial microcantilever produces differential stress, resulting in readily measurable curvatures of the cantilever structure. Depending upon the system studied, there exist two types of gas–solid interaction: bulk-like absorption and surface-like adsorption. The absorption of hydrogen into palladium results in film expansion whose magnitude is governed by hydrogen partial pressure. The bending of a bimaterial microcantilever (palladium/silicon) due to hydrogen absorption depends on the thickness of the palladium film and is reversible but rate limited by a surface barrier. In contrast, the stress induced by adsorption of mercury onto a bimaterial (gold/silicon) cantilever is irreversible at room temperature, is rate limited by surface coverage, and is independent of the gold–film thickness.
High-frequency ultrasound array element using thermoelastic expansion in an elastomeric film
Authors: Buma T. , Spisar M. , O’Donnell M.
Published: July 2002

The thermoelastic effect was used to produce high-frequency, broadband ultrasound in water. A pulsed diode laser, followed by an erbium-doped fiber amplifier, was focused onto a light-absorbing film deposited on a glass substrate. Conversion efficiency was improved by over 20 dB using an elastomeric film instead of a more commonly used metallic one. Radiation pattern measurements show that considerable energy is radiated at +/−45° for frequencies beyond 50 MHz. These results show that the thermoelastic effect can be used to produce phased arrays for high-frequency ultrasound imaging.
Silicon single-electron tunneling device fabricated in an undulated ultrathin silicon-on-insulator film
Authors: Uchida Ken , Koga Junji , Ohba Ryuji , Takagi Shin-ichi , Toriumi Akira
Published: July 2002

We report on a silicon single-electron tunneling device fabricated in an ultrathin (∼3 nm) silicon-on-insulator (SOI) film whose surface is undulated by an alkaline-based solution. The nanometer-scaled undulation in the ultrathin film results in great SOI thickness variations and brings about large electron-potential fluctuations, due to the difference of the quantum confinement effects from one part to another. Consequently, a number of quantum dots are effectively formed in the undulated ultrathin SOI film. This device shows clear Coulomb blockade oscillations at 80 K, as well as nonvolatile single-electron memory functions even at room temperature. The measurements of the undulation with atomic force microscopy reveal that the undulation has two correlation lengths. Based on the analysis of electrical characteristics, it is concluded that the Coulomb blockade oscillations are dominated by a quantum dot formed by the longer-correlation-length undulation and that single-electron memory effects are due to quantum dots formed by the shorter-correlation-length undulation.
A new class of entanglement measures
Authors: Rudolph Oliver
Published: July 2002

We introduce new entanglement measures on the set of density operators on tensor product Hilbert spaces. These measures are based on the greatest cross norm on the tensor product of the sets of trace class operators on Hilbert space. We show that they satisfy the basic requirements on entanglement measures discussed in the literature, including convexity, invariance under local unitary operations and nonincrease under local quantum operations and classical communication.
Thermal expansion of single crystals of the layered compounds TlGaSe2 and TlInS2
Authors: Abdullaev N. A. , Mamedov T. G. , Suleı̆manov R. A.
Published: July 2002

Experimental data are reported from studies of the thermal expansion of single crystals of the layered compounds TlGaSe2 and TlInS2 in the plane of the layers and in the direction perpendicular to the layers in the temperature region 20–300 K. Observed features of the thermal expansion are discussed in the light of the available data on the elastic properties of TlGaSe2 and TlInS2. It is found that the differences in the temperature behavior of the thermal expansion coefficients in the plane of the layers α∥(T) in the TlGaSe2 and TlInS2 crystals are due to the different degree of anisotropy of these crystals and to the different value of the “interlayer” elastic constant C13.
Quantal two-center Coulomb problem treated by means of the phase-integral method. I. General theory
Authors: Athavan N. , Fröman P. O. , Fröman N. , Lakshmanan M.
Published: July 2002

The present paper concerns the derivation of phase-integral quantization conditions for the two-center Coulomb problem under the assumption that the two Coulomb centers are fixed. With this restriction we treat the general two-center Coulomb problem according to the phase-integral method, in which one uses an a priori unspecified base function. We consider base functions containing three unspecified parameters C,C̃, and Λ. When the absolute value of the magnetic quantum number m is not too small, it is most appropriate to choose Λ=|m|≠0. When, on the other hand, |m| is sufficiently small, it is most appropriate to choose Λ=0. Arbitrary-order phase-integral quantization conditions are obtained for these choices of Λ. The parameters C and C̃ are determined from the requirement that the results of the first and the third order of the phase-integral approximation coincide, which makes the first-order approximation as good as possible. In order to make the paper to some extent self-contained, a short review of the phase-integral method is given in the Appendix.
Fermi level dependence of hydrogen diffusivity in GaN
Authors: Polyakov A. Y. , Smirnov N. B. , Pearton S. J. , Ren F. , Theys B. , Jomard F. , Teukam Z. , Dmitriev V. A. , Nikolaev A. E. , Usikov A. S. , Nikitina I. P.
Published: July 2002

Hydrogen diffusion studies were performed in GaN samples with different Fermi level positions. It is shown that, at 350 °C, hydrogen diffusion is quite fast in heavily Mg doped p-type material with the Fermi level close to Ev+0.15 eV, considerably slower in high-resistivity p-GaN(Zn) with the Fermi level Ev+0.9 eV, while for conducting and semi-insulating n-GaN samples with the Fermi level in the upper half of the band gap no measurable hydrogen diffusion could be detected. For these latter samples it is shown that higher diffusion temperature of 500 °C and longer times (50 h) are necessary to incorporate hydrogen to appreciable depth. These findings are in line with previously published theoretical predictions of the dependence of hydrogen interstitials formation in GaN on the Fermi level position.
Diffusion of inert gases in silica sodalite: Importance of lattice flexibility
Authors: Kopelevich Dmitry I. , Chang Hsueh-Chia
Published: July 2002

It is known that some zeolites are able to adsorb sorbates with diameters larger than their pore size. This ability is usually explained by the flexibility of zeolite lattices. In this paper we quantify this explanation by incorporating lattice flexibility into transition state theory to compute transport rates of inert gas molecules (Ne, Ar and Kr) through narrow windows of silica sodalite cages. We find that the diffusion rates obtained with the flexible zeolite lattice are in much better agreement with experiment than those with a rigid lattice, as stretching of the zeolite window considerably reduces the activation barrier.
Dimensionally dependent tensor identities by double antisymmetrization
Authors: Edgar S. Brian , Höglund A.
Published: July 2002

Some years ago, Lovelock showed that a number of apparently unrelated familiar tensor identities had a common structure, and could all be considered consequences in n-dimensional space of a pair of fundamental identities involving trace-free (p,p)-forms where 2p⩾n. We generalize Lovelock’s results, and by using the fact that associated with any tensor in n-dimensional space there is associated a fundamental tensor identity obtained by antisymmetrizing over n+1 indices, we establish a very general “master” identity for all trace-free (k,l)-forms. We then show how various other special identities are direct and simple consequences of this master identity; in particular we give direct application to Maxwell, Lanczos, Ricci, Bel, and Bel-Robinson tensors, and also demonstrate how relationships between scalar invariants of the Riemann tensor can be investigated in a systematic manner.
Effect of length, width, and mode on the mass detection sensitivity of piezoelectric unimorph cantilevers
Authors: Yi Jeong Woo , Shih Wan Y. , Shih Wei-Heng
Published: July 2002

We have investigated both experimentally and theoretically the resonance frequency change of a piezoelectric unimorph cantilever due to the mass loaded at the tip of the cantilever. The piezoelectric cantilever was composed of a lead zirconate titanate (PZT) layer and a stainless steel layer. The dependence of the resonance frequency shift, Δf, with respect to a loaded mass, Δm, on the cantilever length, L, width, w, and the resonance modes was examined. For Δm much smaller than the effective mass of the cantilever, we showed that Δf/Δm increased with an increasing eigen value, νn2, and decreasing length, L, and decreasing width, w, as Δfn/Δm=−(νn2/4π)(1/L3w)(1/0.23612ρ̃)Ẽ/ρ̃, where Ẽ and ρ̃ are the effective Young’s modulus and effective density of the unimorph cantilever that depends on the thickness fraction, Young’s modulus and density of each of the individual layers. Thus, given the same Ẽ and ρ̃ by maintaining the same layer thickness fractions of the individual layers, Δf/Δm is increased by a factor of α−4 when a cantilever is reduced in size by a factor α in proportion in all dimensions. We also showed that the same scaling relationship can be applied to a strip mass loaded at the tip as well as uniformly distributed mass on the cantilever surface provided that the uniformly distributed mass does not form a continuous solid film rigidly bonded to the cantilever surface.
Modeling cross-hatch surface morphology in growing mismatched layers
Authors: Andrews A. M. , Speck J. S. , Romanov A. E. , Bobeth M. , Pompe W.
Published: July 2002

We propose and investigate a model for the development of cross-hatch surface morphology in growing mismatched layers. The model incorporates two important elements: (i) strain relaxation due to dislocation glide in the layer (film) interior that is also associated with misfit dislocation formation at the film/substrate interface and (ii) lateral surface transport that eliminates surface steps that originated from dislocation glide. A combination of dislocation-assisted strain relaxation and surface step flow leads to the appearance of surface height undulations during layer growth. A Monte Carlo simulation technique was applied to model dislocation nucleation events in the course of strain relaxation. The simulation was used to model the influence of dislocations on film surface height profiles. The surface height displacement was calculated from the analytic elasticity solutions for edge dislocations near a free surface. The results of the modeling predict that the average amplitude of the surface undulations and their apparent wavelength both increase with increasing film relaxation and film thickness. The developed cross-hatch pattern is characterized by an atomically smooth but mesoscopically (lateral dimensions ∼0.1–10 μm) rough surface morphology. The conclusions of the model are in agreement with atomic force microscopy observations of cross-hatch surface relief in In0.25Ga0.75As/GaAs samples grown well beyond the critical thickness for misfit dislocation formation.
Kinetics of the creep deformation jump stimulated by the superconducting transition in β-tin
Authors: Soldatov V. P. , Natsik V. D. , Kirichenko G. I.
Published: July 2002

The kinetics of the deformation increment stimulated by the superconducting transition in β-tin is investigated. Experiments are done on single crystals of high purity strained in the transient creep regime at a temperature of 1.6 K. It is found that the creep after the N–S transition occurs in stages, and the features of the strain-rate versus strain curves are analyzed within each stage. An interpretation of the experimental results is proposed, based on a kink mechanism for the motion of dislocations through the Peierls barriers with allowance for the influence of quantum fluctuations and electron drag on the nucleation of kinks.
Laser ablation of dielectrics with temporally shaped femtosecond pulses
Authors: Stoian R. , Boyle M. , Thoss A. , Rosenfeld A. , Korn G. , Hertel I. V. , Campbell E. E. B.
Published: July 2002

A significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.
Entropic effects on the structure of Lennard-Jones clusters
Authors: Doye Jonathan P. K. , Calvo Florent
Published: July 2002

We examine in detail the causes of the structural transitions that occur for those small Lennard-Jones clusters that have a nonicosahedral global minima. Based on the principles learned from these examples, we develop a method to construct structural phase diagrams that show in a coarse-grained manner how the equilibrium structure of large clusters depends on both size and temperature. The method can be augmented to account for anharmonicity and quantum effects. Our results illustrate that the vibrational entropy can play a crucial role in determining the equilibrium structure of a cluster.
Dynamics of dendrimers and of randomly built branched polymers
Authors: von Ferber C. , Blumen A.
Published: July 2002

We focus on the dynamical properties of dendrimers and of randomly built branched polymers, which allows us to assess theoretically the role of disorder on the relaxation forms. We model the random polymers through a stochastic growth algorithm. Our studies are carried out both in the Rouse and also in the Zimm framework; the latter accounts for hydrodynamic interactions. Moreover, we also mimic the local geometry by imposing conditions on the average values of the angles between neighboring segments. Excluded volume interactions, however, are neglected throughout. The storage G′(ω) and the loss G″(ω) moduli, which we calculate, turn out to depend more on the hydrodynamic and the angular restrictions than on randomness. Furthermore, we find that both the randomness and the angular restrictions slow down the relaxation. Given that G′(ω), G″(ω) and also C(t), a function related to the radius of gyration, are all connected to the relaxation function G(t), a fact which we recall, we also calculate numerically G(t) and C(t); moreover we fit, following previous works, C(t) to stretched-exponential forms. Interestingly, it appears that from all functions considered G(t) is most sensitive to disorder.
Fishbone mode in “potato” regime
Authors: Marchenko V. S.
Published: July 2002

The resonance destabilization of the m=1/n=1 internal kink by trapped energetic ions executing potato orbits is considered. Abrupt stabilization is found for δpα>rs, with δpα being the potato width at injection energy and rs the radius of the q=1 surface. The magnetic well in spherical tokamaks stabilizes fishbones even with the finite orbit width effect included, increasing threshold by a factor ∼α, with α≫1 being the magnetic well curvature parameter.
Fundamentals of synchronization in chaotic systems, concepts, and applications
Authors: Pecora Louis M. , Carroll Thomas L. , Johnson Gregg A. , Mar Douglas J. , Heagy James F.
Published: July 2002

The field of chaotic synchronization has grown considerably since its advent in 1990. Several subdisciplines and “cottage industries” have emerged that have taken on bona fide lives of their own. Our purpose in this paper is to collect results from these various areas in a review article format with a tutorial emphasis. Fundamentals of chaotic synchronization are reviewed first with emphases on the geometry of synchronization and stability criteria. Several widely used coupling configurations are examined and, when available, experimental demonstrations of their success (generally with chaotic circuit systems) are described. Particular focus is given to the recent notion of synchronous substitution—a method to synchronize chaotic systems using a larger class of scalar chaotic coupling signals than previously thought possible. Connections between this technique and well-known control theory results are also outlined. Extensions of the technique are presented that allow so-called hyperchaotic systems (systems with more than one positive Lyapunov exponent) to be synchronized. Several proposals for “secure” communication schemes have been advanced; major ones are reviewed and their strengths and weaknesses are touched upon. Arrays of coupled chaotic systems have received a great deal of attention lately and have spawned a host of interesting and, in some cases, counterintuitive phenomena including bursting above synchronization thresholds, destabilizing transitions as coupling increases (short-wavelength bifurcations), and riddled basins. In addition, a general mathematical framework for analyzing the stability of arrays with arbitrary coupling configurations is outlined. Finally, the topic of generalized synchronization is discussed, along with data analysis techniques that can be used to decide whether two systems satisfy the mathematical requirements of generalized synchronization.
Analytical solutions describing the phase separation driven by a free energy functional containing a long-range interaction term
Authors: Ohnishi Isamu , Nishiura Yasumasa , Imai Masaki , Matsushita Yushu
Published: July 2002

We are primarily concerned with the variational problem with long-range interaction. This functional represents the Gibbs free energy of the microphase separation of diblock copolymer melts. The critical points of this variational problem can be regarded as the thermodynamic equilibrium state of the phase separation phenomenon. Experimentally it is well-known in the diblock copolymer problem that the final equilibrium state prefers periodic structures such as lamellar, column, spherical, double-diamond geometries and so on. We are interested in the characterization of the periodic structure of the global minimizer of the functional (corresponding to the strong segregation limit). In this paper we completely determine the principal part of the asymptotic expansion of the period with respect to ε (interfacial thickness), namely, we estimate the higher order error term of the period with respect to ε in a mathematically rigorous way in one space dimension. Moreover, we decide clearly the dependency of the constant of proportion upon the ratio of the length of two homopolymers and upon the quench depth. In the last section, we study the time evolution of the system. We first study the linear stability of spatially homogeneous steady state and derive the most unstable wavelength, if it is unstable. This is related to spinodal decomposition. Then, we numerically investigate the time evolution equation (the gradient flow of the free energy), and see that the free energy has many local minimizers and the system have some kind of sensitivity about initial data.
The high-speed single-pass bin sort
Authors: Cant G. P.
Published: July 2002

The little-known bin sort will perform more quickly than other sorting routines. With an address calculation formula that is chosen to approximate the cumulative distribution, it can readily handle cases for which it has the reputation of being ill-suited.
Numeric precision in FORTRAN computing
Authors: Meredith Roger W.
Published: July 2002

This article compares both real and complex outputs from sizeable numeric computations using identical code on several computer systems. The digital signal processing technique known as the modified covariance method was used as the computational engine. It is a recursive algorithm for solving the covariance equations of a linear predictor that seeks to predict an input signal by a linear combination of past signal samples. Single precision and double precision results are presented but the study focuses primarily on differences between the VAX Fortran 4.8 and MacFortran/020 compilers. Differences in the first digit for single precision arithmetic were found and double precision differences occurred in the eighth digit. Arithmetic with complex data types was found to be less precise than with real data types. Although differences exist among various computer systems, they all show the same order of magnitude accuracy with respect to CRAY-YMP results. The algorithm used here required a double precision implementation to obtain agreement between different computer systems.
Real-time physics data-visualization system using Performer
Authors: Mitchell Chris , Gekelman Walter
Published: July 2002

A data visualization system geared toward large (gigabyte or greater-than-gigabyte) time-varying scientific data sets has been written. The system is a distributed multiprocessed application that employs techniques used both in the video-game industry and military flight simulators to achieve real-time frame rates. On the computational side, the system calculates vector fields, streamlines, isosurfaces, and cutplanes in real time. The graphics component employs strategic visual database decomposition and levels of detail to maintain a fast frame rate. This allows one to navigate through the data sets as they evolve temporally. © 1998 American Institute of Physics.
Shock-wave initiation of heterogeneous reactive solids
Authors: Johnson J. N. , Tang P. K. , Forest C. A.
Published: July 2002

Shock-wave initiation of solid explosives depends on localized regions of high temperature (hot spots) created by heterogeneous deformation in the vicinity of various defects. Current mathematical models of shock initiation tend to fall into two broad categories: (1) thermodynamic-state-dependent reaction-rate models, and (2) the continuum theory of multiphase mixtures. The level of generality possessed by (1) appears to be insufficient for representation of observed initiation phenomena, while that of (2) may exceed necessary requirements based on present measurement capabilities. As a means of bridging the gap between these two models, we present an internal-state-variable theory based on elementary physical principles, relying on specific limiting cases for the determination of functional forms. The appropriate minimum set of internal-state variables are the mass fraction of hot spots  μ, their degree of reaction  f, and their average creation temperature θ. The overall reaction rate λ̇, then depends on  μ,  f, and θ in addition to the usual macroscopic thermodynamic variables (current state as well as their history). Two specific forms of this set of equations are applied to time-resolved shock-initiation data on PBX-9404. Numerical solution is achieved by the method of characteristics for rate-dependent chemical reaction. Additional questions such as the effect of thermal equilibrium between phases (solid reactants and gaseous products) on the theoretical results are discussed quantitatively.
Mechanisms of deposition and etching of thin films of plasma-polymerized fluorinated monomers in radio frequency discharges fed with C2F6-H2 and C2F6-O2 mixtures
Authors: d’Agostino Riccardo , Cramarossa Francesco , Illuzzi Francesca
Published: July 2002

Deposition and etching of thin plasma-polymerized fluorinated monomers have been studied in discharges fed with C2F6-H2 and C2F6-O2 mixtures, respectively. A parallel plate reactor with thermostated electrodes has been utilized for the present study. The experiments have allowed us to ascertain the role of positive ions and of radicals, as well as the effect of pressure and substrate temperature, on the mechanism of deposition. The mechanism of etching has also been investigated, and it has been found that both O and F atoms contribute to the process through overall first-order kinetics. Conditions for a selective etching of noncross-linked films with respect to cross-linked ones have also been found.
Analysis of the interaction of a laser pulse with a silicon wafer: Determination of bulk lifetime and surface recombination velocity
Authors: Luke Keung L. , Cheng Li-Jen
Published: July 2002

The decay of excess minority carriers produced in a silicon wafer of thickness d by a laser pulse is analyzed. A comprehensive theory based on this analysis is presented for the determination of bulk lifetime τb and surface recombination velocity S. It is shown that, starting with an exponential spatial profile, the carrier profile assumes a spatially symmetrical form after approximately one time constant of the fundamental mode of decay. Expressions for the spatial average of the carrier density as a function of time are derived for three temporal laser pulse shapes: impulse, square, and Gaussian. Particular attention is paid to the time constants of the fundamental and higher modes of decay. The ratios of the time constants of the higher modes to the fundamental mode, as well as the time constant of the fundamental mode, are presented over wide ranges of values of S and d. Two complementary methods applicable for values of the product Sd≳40 cm2/s are proposed to determine τB and S, without having to invoke the usual stringent requirement of either S→∞ or d→∞, and thus permit the determination of τb and S for a greater variety of wafer characteristics. For Sd≲40 cm2/s, a two-wafer method is developed to determine τb and S; it is also shown that the requirement of d/τb ≳20S is sufficient to adequately guarantee that the asymptotic value of the instantaneous observed lifetime differs from the bulk lifetime by no more than 10%.
Initial stages of epitaxial growth of GaAs on (100) silicon
Authors: Biegelsen D. K. , Ponce F. A. , Smith A. J. , Tramontana J. C.
Published: July 2002

Direct observations of early stages of molecular-beam epitaxial growth of GaAs on oriented and vicinal (100) Si surfaces are presented. Cross-sectional transmission electron microscopy and plan view scanning electron microscopy images directly reveal three-dimensional island growth for substrate temperatures above 300 °C. Island size, island spacing, surface morphology, and stacking fault defect spacing all increase with substrate temperature for fixed Ga and As fluxes. Below 300 °C, 7-nm-thick films are continuous and uniform. Films deposited on surfaces tilted from (100) coalesce anisotropically with respect to the tilt axis.
Generation of misfit dislocations in semiconductors
Authors: Marée P. M. J. , Barbour J. C. , van der Veen J. F. , Kavanagh K. L. , Bulle-Lieuwma C. W. T. , Viegers M. P. A.
Published: July 2002

The acting slip mechanism for the generation of misfit dislocations in diamond-type–semiconductor heterostructures is investigated with transmission electron microscopy. It is shown that dissociation of the 60°-mixed dislocations can lead to a difference in strain accommodation for tensile and compressive strain. A strain/thickness relation is obtained from the energy expression for nucleation of half-loops. This relation is compared with other theoretical relations and with experimental strain data for Si/GaP(001) and In0.07Ga0.93As/GaAs(001) , measured with transmission electron microscopy and ion blocking.
Observation of magnetic forces by the atomic force microscope
Authors: Sáenz J. J. , García N. , Grütter P. , Meyer E. , Heinzelmann H. , Wiesendanger R. , Rosenthaler L. , Hidber H. R. , Güntherodt H.-J.
Published: July 2002

We present a new way to observe the surface domain distribution of a magnetic sample at a submicrometer scale. This magnetic microscopy is based on the idea of measuring magnetic forces with the recently developed atomic force microscope (AFM). We study the magnetic forces involved in the interaction between a single-domain microtip and the sample surface magnetic domains. The influence of the experimental conditions on the performance of the AFM as a magnetic profiling device is also discussed. Preliminary experimental results are reported.
Predicting thin-film stoichiometry in reactive sputtering
Authors: Berg S. , Larsson T. , Nender C. , Blom H-O.
Published: July 2002

The electrical, optical, and mechanical properties of a compound film depend strongly on the composition of the film. Therefore, it is interesting to study a wide variety of compositions of many new compound materials. Reactive sputtering is a widely used technique to produce compound thin films. With this technique it is possible to fabricate thin films with different compositions. However, it has not yet, to any great extent, been possible to predict the composition of the sputtered film. In this article we will present a model that enables us to predict both sputtering rate and film composition during reactive sputtering. The results point out that there exists a very simple linear relationship between processing parameters for maintaining constant thin-film composition in the reactive sputtering process. Based on these results, it is possible for the first time to combine information of both sputtering rate and film composition into the same graphical representation. Access to this new and simple graphical representation may eliminate much of the ‘‘trial and error’’ work that earlier has been associated with the reactive sputtering process.
Magnetic properties and microstructure of iron-based rare-earth magnets with low-temperature coefficients
Authors: Zhou S. Z. , Guo C. , Hu Q.
Published: July 2002

The influence of Dy substitution for Nd on magnetic properties, temperature characteristics, and microstructure of (FeCo)NdB magnets has been studied. The Dy appears mainly in the tetragonal matrix phase and in the Nd-rich phase. The Dy in the tetragonal matrix phase increases the anisotropy, and twin boundaries and convoluted grain boundaries appear. These phenomena appear to be related to the increase in coercivity. In addition to the Nd(FeCo)2 (Laves) phase in the alloys with high Co and Dy content, a new phase (NdDy)(CoFe)5.2 appears. This phase causes coercivity to decrease, but may improve the temperature coefficient of the alloys.
Growth kinetics and step density in reflection high-energy electron diffraction during molecular-beam epitaxy
Authors: Clarke Shaun , Vvedensky Dimitri D.
Published: July 2002

The kinetics of molecular-beam epitaxy are examined by means of Monte Carlo simulations in combination with a new approach for monitoring surface growth, i.e., by calculating the evolution of the surface step density. The evolution of the step density is shown to have a remarkable correspondence to that of the measured reflection high-energy electron diffraction (RHEED) specular spot intensities for III-V semiconductor compounds. We study growth in a variety of systems, including flat and stepped surfaces, as a function of substrate temperature and draw several conclusions concerning the relation between RHEED measurements, kinetics, and growth quality. The range of validity of the kinematic approach to RHEED is discussed and the importance of multiple scattering in the high step density regime is highlighted.
Observation of E0 and E1 transitions in AlGaP alloys by electrolyte electroreflectance
Authors: Rodríguez J. M. , Armelles G. , Salvador P.
Published: July 2002

The optical properties of AlxGa1−xP alloys of various compositions have been investigated by electrolyte electroreflectance. E0 and E1 optical transitions have been observed for the first time in the spectral region between 3000 and 4500 Å. Both empirical relationships, E0=2.78+1.05x and E1=3.77+0.26x, between the transition energies and the alloy composition were found. The extrapolated values for x=1 were the first semiempirical values for the E0 and E1 transition of AlP at room temperature. The indirect energy gap of the sample with x=0.30 was obtained from photoelectrochemical measurements to be 2.43 eV, a value slightly higher than that reported in the literature for the same composition.