Symmetric Directions Research Articles

Spatial distribution of high-energy electron emission from water plasmas produced by femtosecond laser pulses.

High energy electrons emitted by water plasmas produced by a single or a multiple laser pulse are investigated. The multipulse mode greatly enhances the generation and the temperature of hot electrons. Directional emission of high energy electrons over 25 keV is observed in two symmetric directions with respect to the laser axis and at 46 degrees from the directions of the laser electric field. Two-dimensional particle-in-cell simulations reproduce well the experimental results and indicate that the acceleration mechanism of the high energy electrons is due mainly to the resonance absorption at the edge of the spherical droplets formed by the leading pulse.

Atomic and electronic structures of Si( [formula omitted])1×1-Sb surface: core-level shifts and surface states

Atomic and electronic structures of a Si(1 1 3)1×1-Sb surface are investigated by synchrotron-radiation photoelectron spectroscopy. The Si 2p and Sb 4d core levels as well as the valence band dispersions are measured. All the components of the Si 2p core level spectrum related to the clean surface have been disappeared and one Sb-induced component could be identified at a higher binding energy of about +0.17 eV with respect to the bulk component. The Sb 4d core level spectrum is well represented by one doublet component, characteristic of Sb atoms adsorbed in an unique environment. For the electronic structure of the 1×1-Sb surface, single surface state positioned at 1.1 eV below the Fermi level at Γ, is recognized by angle-resolved photoemission spectroscopy and the dispersions has been mapped along the main symmetric directions in the surface Brillouin zone. The state shows a strong parabolic dispersion of about 0.95 eV along the [1 −1 0] direction while, in the [3 3 −1] direction, the state showed only small dispersion of about 0.35 eV. The 1×1-Sb surface shows a strong chemical passivation, which is due to the absence of the surface dangling bonds. In order to explain the results of the core level shifts, the band dispersions, and the surface inertness, simple structure model is suggested.

Angle-resolved photoemission data and quasiparticle spectra in antiferromagnetic insulators Sr2CuO2Cl2 and Ca2CuO2Cl2

We have analyzed the quasiparticle dispersion and angle-resolved photoemission spectroscopy (ARPES)-spectral density for ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ antiferromagnetic insulators along basic symmetric directions of the Brillouin zone (BZ) in the framework of a generalized tight binding method (GTBM) with account for strong electron correlations. The quasiparticle dispersion is in a good agreement with ARPES data. At the top of the valence band we found a narrow impuritylike virtual level with the spectral weight proportional to the concentration of spin fluctuations. A pseudogap between the virtual level and the top of the valence band has dispersion similar to ``remnant Fermi surface'' in ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and to a pseudogap in the underdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+d}(\mathrm{Dy})$ (Bi2212) samples. A calculated parity of the polarized ARPES spectra in $\ensuremath{\Gamma},M,X$ points in the AFM phase is even with regard to relative magnitudes of the partial contributions from different orbitals to the total ARPES-spectral density. Possible conditions of an observation for the different partial contributions in the polarized ARPES experiments are determined also.

An ab initio direct-trajectory study of the kinetic isotope effect on the bifurcating reaction

The isotope effects on the bifurcating reaction path of the isomerization reaction, H3CO→H2COH (stereoisomers of P and P*), have been investigated recently [J. Chem. Phys. 113, 477 (2000)], and it was found that the valley-ridge inflection point appears in the totally symmetric direction relative to the intrinsic reaction coordinate in the deuterium-substituted system. In the present report, the kinetic isotope effect on this reaction has been studied using an ab initio direct-trajectory method at the computational level of the complete active space self-consistent field method with 6-31G(d,p) basis sets. The branching ratio to the respective stereoisomers has been investigated in terms of distributions of several coordinates along the trajectories. The energy landscape for H3CO is also described in terms of several structures and the respective permutational isomers.

Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire.

ABSTRACTProperties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on c-plane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≍ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9×108 - 2 ×109 cm−2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≍ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors.

Experimental determination of the valence band structure of the InxGa1-x-As(001) surface

InxGaj1-xAs layers were grown on GaAs(00l) substrates by molecular beam epitaxy. The structure and quality of the surface and the layer thickness were monitored in situ by dynamical reflection high-energy electron diffraction (RHEED oscillations). Angle-resolved photoemission spectroscopy studies using He-I radiation were performed on the epitaxial layers. The energy-distribution curves of the photoelectrons were determined along the high symmetric directions in the surface Brillouin zone. The experimental band structure of InxGa1-xAs was determined with the help of the experimental and theoretical band structures of GaAs and In As.

Nonlinear regimes of resonance precession of the magnetization in a (111) iron-garnet film

The special features of precessional motion under ferromagnetic resonance conditions with perpen-dicular magnetization of the film are investigated on the basis of a numerical solution of the equations of motion of magnetization in a (111) type iron garnet film. Several nonlinear magnetization precession regimes exist for fixed values of the crystallographic and induced anisotropies. Depending on the value of the magnetizing field and the amplitude of the microwave field, precession occurs around the normal to the film with the third harmonic of the fundamental frequency making a small or large contribution to the nutation motion and with a small or large amplitude of the precession angle. Precession regimes around one of three symmetric directions different from the direction of the normal are possible. Narrow ranges of the static field, where dynamical bistability and regimes with a period which is a multiple of the period of the microwave field are realized, exist.

Electron microscopy analysis of decagonal quasicrystals in the Al-Cu-Co-Si system

Abstract Large needle-shaped grains of a decagonal quasicrystal in the Al–Cu–Co–Si quaternary system were prepared by slow cooling from the melt. Scanning electron microscopy, X-ray diffraction and convergent-beam electron diffraction patterns confirmed that the needles have a decagonal quasicrystal structure. Thin slices normal to the tenfold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots and in between weaker diffraction spots as well as satellite spots were observed in the tenfold diffraction patterns. Bright-field and dark-field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. Dark contrast at the point of discontinuity of the fringes are observed along a particular direction. High-resolution electron microscopy images also indicated features corresponding to the striation contrast. The striation contrast is shown to be associated with a phason disorder boundary in the partially ordered decagonal phase. Satellite spots are attributed as a characteristic feature of the partially ordered decagonal superlattice structure, which is an intermediate state between a fully disordered and fully ordered structure of the decagonal phase. The atomic rearrangement or phason displacements in certain symmetric directions (along the pentagrids or Ammann lines in the structure) give rise to the striation contrast observed in the images. The evolution of domains consisting of periodic and quasiperiodic parallel boundaries in localized regions can be interpreted as one of the signatures of an intermediate structural state prior to the formation of a fully ordered decagonal lattice.

Electron microscopy analysis of decagonal quasicrystals in the Al–Cu–Co–Si system

Abstract Large needle-shaped grains of a decagonal quasicrystal in the Al–Cu–Co–Si quaternary system were prepared by slow cooling from the melt. Scanning electron microscopy, X-ray diffraction and convergent-beam electron diffraction patterns confirmed that the needles have a decagonal quasicrystal structure. Thin slices normal to the tenfold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots and in between weaker diffraction spots as well as satellite spots were observed in the tenfold diffraction patterns. Bright-field and dark-field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. Dark contrast at the point of discontinuity of the fringes are observed along a particular direction. High-resolution electron microscopy images also indicated features corresponding to the striation contrast. The striation contrast is shown to be associated with a phason disorder boun...

Isotope effect on bifurcating reaction path: Valley–ridge inflection point in totally symmetric coordinate

The isotope effects on bifurcating reaction paths have been studied for two reactions, H3CO→H2COH and HF+C2H4→C2H5F, both of which have a valley–ridge inflection (VRI) point related to nontotally symmetric directions on the symmetry-conserved intrinsic reaction coordinate (IRC). As the result of the VRI occurrence, the IRC connects a reactant minimum, the transition state, and another first-order saddle point. The deuterium substitution of one or two hydrogen atoms in molecular systems reduces the symmetry of the IRC, resulting in the direct linkage of the transition state and a product minimum through the IRC. In the above two reactions with deuterium substitution, it is found that the VRI occurs relative to the totally symmetric directions. Analyses of the steepest descent paths that start from a zero-point energy region around the VRI point demonstrate that those paths bifurcate toward two different product minima.

Kerr–Schild Symmetries

We study continuous groups of generalized Kerr-Schild transformations and the vector fields that generate them in any n-dimensional manifold with a Lorentzian metric. We prove that all these vector fields can be intrinsically characterized and that they constitute a Lie algebra if the null deformation direction is fixed. The properties of these Lie algebras are briefly analyzed and we show that they are generically finite-dimensional but that they may have infinite dimension in some relevant situations. The most general vector fields of the above type are explicitly constructed for the following cases: any two-dimensional metric, the general spherically symmetric metric and deformation direction, and the flat metric with parallel or cylindrical deformation directions.

The subdominant hand increases in the efficacy of voluntary alterations in bimanual coordination.

Subjects performed a bimanual circle-tracing task in time with an auditory metronome while restricted to moving with either proximal or distal musculature of the upper limb. Patterns were made in symmetric or asymmetric directions with respect to the midline. Symmetric patterns were more stable than asymmetric patterns. In response to a visual stimulus, subjects reversed the direction of one limb. Unwanted disruptions (momentary or lasting reversals) in the limb contralateral to the reversing limb were observed in 48% of trials. Incidence of disruption was equivalent between postures, but occurred more frequently when the dominant hand reversed direction. This result is consistent with anisotropy in coupling between hands and reveals a unifying constraint between spontaneous and intentional dynamics in bimanual coordination.

Controllability and motion algorithms for underactuated Lagrangian systems on Lie groups

We provide controllability tests and motion control algorithms for underactuated mechanical control systems on Lie groups with Lagrangian equal to kinetic energy. Examples include satellite and underwater vehicle control systems with the number of control inputs less than the dimension of the configuration space. Local controllability properties of these systems are characterized, and two algebraic tests are derived in terms of the symmetric product and the Lie bracket of the input vector fields. Perturbation theory is applied to compute approximate solutions for the system under small-amplitude forcing; in-phase signals play a crucial role in achieving motion along symmetric product directions. Motion control algorithms are then designed to solve problems of point-to-point reconfiguration, static interpolation and exponential stabilization. We illustrate the theoretical results and the algorithms with applications to models of planar rigid bodies, satellites and underwater vehicles.

Electron microscopy study of striation contrast in Al-Cu-Co-Si decagonal quasicrystals

Detailed transmission electron microscopy study was carried out in single crystals of a decagonal phase in the Al-Cu-Co-Si quaternary system. X-ray diffraction and convergent beam electron diffraction patterns of the powder samples confirmed the structures to be decagonal quasicrystals. No microcrystalline nor crystalline phases could be identified. Thin slices normal to the 10-fold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots were observed in the diffraction patterns. Bright field images and dark field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. The striation contrast appears to be originating from anti-phase boundary (APB) in the decagonal superstructures. The diffuse streaks seem to be a characteristic feature of a partially ordered decagonal superlattice structure. The atomic rearrangement or phasonic movement in certain symmetric directions along the pentagrids or Ammann lines in the structure has obviously caused the type of contrast observed in the images. The evolution of rhombic domains consisting of APBs in localized regions can be understood as one of the signature of an intermediate structural state formed prior to a superstructure formation.

Elastoplastic analysis of inclusion reinforced structures

An analytical model for assessing the global elastoplastic behaviour of inclusion-reinforced materials is presented in this contribution. It is based upon a description of the reinforced material as a two-phase composite system, namely a matrix material and the reinforcements which are assumed to behave as tensile-compressive load carrying elements. An anisotropic elastoplastic constitutive law exhibiting work-hardening is then derived in an explicit form. It involves a number of hardening parameters equal to the number of reinforcing directions. Such a model, which is readily implementable in a finite element computer code, is applied to the numerical simulation of the settlement of a shallow strip footing resting upon a soil reinforced in two symmetric directions (“micropiling technique”). The load-settlement curve predicted from using the work-hardening model is finally compared with that deduced from a previously-adopted elastic perfectly plastic schematization of the reinforced soil.

Inelastic seismic response of one-way plan-asymmetric systems under bi-directional ground motions

The static design requirements of some seismic codes, such as the Eurocode 8 and—in most cases—the Uniform Building Code, to allow for the effects of earthquake excitation acting in a direction other than the principal axes of the structure do not apply to one-way asymmetric systems. Therefore, with some exceptions, no specific provisions are considered for such systems to cover effects of structural asymmetry on the behaviour of elements located along the symmetric system direction. Aimed towards fulfilling this need, in this paper, a wide parametric study of the inelastic response of one-way asymmetric systems designed according to Uniform Building Code is carried out, considering two-component earthquake excitations. The analyses show that the maximum ductility demands on elements aligned along the asymmetric system direction are very close to, and even lower than, those obtained for symmetric reference systems. Conversely, the symmetric direction elements undergo significantly larger inelasticity than if they were located in symmetric reference systems. Subsequently, the overstrength needed by the symmetric direction elements to prevent such additional ductility demands for several stiffness and plan configurations is quantified. It is concluded that one-way asymmetry should be considered by seismic codes as an intrinsic system property, thus implying that specific provisions should be included for designing elements located along the symmetric system direction, in addition to those currently subscribed to design the asymmetric direction elements. © 1998 John Wiley & Sons, Ltd.

Procedure to Determine the Structure of Three-Horned Multishell Fullerene

Abstract Fullerenes have many families such as simple ball-shaped fullerenes, nanotubes, and onions. Additionally torus, Y-shaped particle, and graphitic network with periodic holes or dimples have been theoretically anticipated. The presence of various shapes in fullerenes is mainly attributed to the combination of five-membered and seven-membered rings in addition to six-membered rings composing ordinary graphite crystal. The five-membered rings are necessary for positive curvature of the fullerenes, while the seven-membered rings for negative curvature. Experimentally, the negative curvature has been restricted so far to the joint between cone and tube in nanotubes and the spiral tubes. We have found a new type of multishell fullerenes with negative curvature which consist of three horns protrude into three-fold symmetric directions, referred as three-horned multishell fullerene (THF) in this report, using a high resolution transmission electron microscope (TEM). Here, we first describe the formation briefly and secondly concerned with procedure to determine the structure of the THF.

Approaches to bifurcating reaction path

The intrinsic reaction path (IRP) often becomes unstable relative to some nontotally symmetric direction orthogonal to the path through a valley–ridge inflection point. We investigate geometric characters of the potential energy surface around the valley–ridge inflection boundary, and propose some ideas to determine a bifurcating reaction path, or to give a two-dimensional potential energy surface which connects bifurcating point and product regions. As a demonstration, bifurcating reaction paths are calculated for the isomerization reaction of methoxy radical (H3CO→H2COH) by the unrestricted Hartree–Fock (UHF) method.

Traveling-wave states and secondary instabilities in optical parametric oscillators.

A complex order-parameter description of pattern formation in large aspect ratio optical parametric oscillators (OPOs) with flat end reflectors and uniform pumping is presented starting from the mean-field model of the OPO equations [G.-L. Oppo et al., Phys. Rev. A 49, 2028 (1994)]. It is shown that, in the nondegenerate case, the full OPO equations have an exact continuum family of traveling-wave (TW) solutions, which are preferred to standing-wave (SW) states found in the degenerate case. These solutions correspond to an off-axis emission for both signal and idler fields along two symmetric directions to satisfy momentum conservation in the parametric conversion process. Stability of TW versus SW solutions is investigated by deriving two coupled Newell-Whitehead-Segel equations describing the growth of SW or TW close to threshold. Analytical expressions for long-wavelength phase instabilities of the TW states above threshold are obtained from the coefficients of a Cross-Newell phase equation, and are shown to be the same for OPOs with high or low finesse for the pump field. By direct linear stability analysis of the TW solutions, it is also shown that the appearance of amplitude instabilities may reduce the region of stable TW states in the case of OPOs with a high finesse for the pump field. \textcopyright{} 1996 The American Physical Society.

Two-dimensional periodic waves in shallow water. Part 2. Asymmetric waves

We demonstrate experimentally the existence of a family of gravity-induced finiteamplitude water waves that propagate practically without change of form in shallow water of uniform depth. The surface patterns of these waves are genuinely two-dimensional, and periodic. The basic template of a wave is hexagonal, but it need not be symmetric about the direction of propagation, as required in our previous studies (e.g. Hammack et al. 1989). Like the symmetric waves in earlier studies, the asymmetric waves studied here are easy to generate, they seem to be stable to perturbations, and their amplitudes need not be small. The Kadomtsev–Petviashvili (KP) equation is known to describe approximately the evolution of waves in shallow water, and an eight-parameter family of exact solutions of this equation ought to describe almost all spatially periodic waves of permanent form. We present an algorithm to obtain the eight parameters from wave-gauge measurements. The resulting KP solutions are observed to describe the measured waves with reasonable accuracy, even outside the putative range of validity of the KP model.