The Nature of [Ar III] Bright Knots in the Crab Nebula

The kinematic and morphological properties of a string of [Ar III] bright knots in the Crab Nebula are examined using 1994 - 1999 HST WFPC-2 images of the remnant. We find that five southern [Ar III] bright knots exhibit ordinary radial motions away from the nebula's center of expansion with magnitudes consistent with their projected radial displacements. These results do not support the suggestion by MacAlpine et al.(1994) that these knots might be moving rapidly away from the Crab pulsar due to a collimated wind. The HST images also do not show that the [Ar III] knots have unusual morphologies relative to other features in the remnant. Our proper motion results, when combined with radial velocity estimates, suggest these knots have relatively low space velocities implying relatively interior remnant locations thus placing them closer to the ionizing radiation from the Crab's synchrotron nebula. This might lead to higher knot gas temperatures thereby explaining the knots' unusual line emission strengths as MacAlpine et al.(1994) suspected.Comment: 11 pages including three figures. Submitted to the Astronomical Journa

Localized Coating Removal Using Plastic Media Blasting

USBI, a Division of United Technologies/ is responsible for the assembly, checkout and refurbishment of the structural, guidance and recovery components of the Solid Rocket Booster (SRB) as part of the NASA Space Transportation system/ Space Shuttle. The work is performed at Kennedy Space Center/ Florida and the contract is administered by Marshall Space Flight Center (MSFC) in Huntsville/ Alabama. Figure 1 shows the SRB and associated hardware that USBI is responsible for. Recently/ a considerable effort was made to qualify the use of Plastic Media Blasting (PMB) for safely and effectively removing paint and other coatings from SRB aluminum structures. As a result of the effort an improvement was made in the design of surface finishing equipment for processing flight hardware/ in addition to a potentially patentable idea on improved plastic media composition

Electronic properties of silica nanowires

Thin nanowires of silicon oxide were studied by pseudopotential density functional electronic structure calculations using the generalized gradient approximation. Infinite linear and zigzag Si-O chains were investigated. A wire composed of three-dimensional periodically repeated Si4O8 units was also optimized, but this structure was found to be of limited stability. The geometry, electronic structure, and Hirshfeld charges of these silicon oxide nanowires were computed. The results show that the Si-O chain is metallic, whereas the zigzag chain and the Si4O8 nanowire are insulators

Transferable Pair Potentials for CdS and ZnS Crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures, as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials

First Principles Studies on 3-Dimentional Strong Topological Insulators: Bi2Te3, Bi2Se3 and Sb2Te3

Bi2Se3, Bi2Te3 and Sb2Te3 compounds are recently predicted to be 3-dimentional (3D) strong topological insulators. In this paper, based on ab-initio calculations, we study in detail the topological nature and the surface states of this family compounds. The penetration depth and the spin-resolved Fermi surfaces of the surface states will be analyzed. We will also present an procedure, from which highly accurate effective Hamiltonian can be constructed, based on projected atomic Wannier functions (which keep the symmetries of the systems). Such Hamiltonian can be used to study the semi-infinite systems or slab type supercells efficiently. Finally, we discuss the 3D topological phase transition in Sb2(Te1-xSex)3 alloy system.Comment: 8 pages,17 figure

Temperature dependence of the electronic structure of semiconductors and insulators

The renormalization of electronic eigenenergies due to electron-phonon coupling is sizable in many materials with light atoms. This effect, often neglected in ab-initio calculations, can be computed using the perturbation-based Allen-Heine-Cardona theory in the adiabatic or non-adiabatic harmonic approximation. After a short description of the numerous recent progresses in this field, and a brief overview of the theory, we focus on the issue of phonon wavevector sampling convergence, until now poorly understood. Indeed, the renormalization is obtained numerically through a q-point sampling inside the BZ. For q-points close to G, we show that a divergence due to non-zero Born effective charge appears in the electron-phonon matrix elements, leading to a divergence of the integral over the BZ for band extrema. Although it should vanish for non-polar materials, unphysical residual Born effective charges are usually present in ab-initio calculations. Here, we propose a solution that improves the coupled q-point convergence dramatically. For polar materials, the problem is more severe: the divergence of the integral does not disappear in the adiabatic harmonic approximation, but only in the non-adiabatic harmonic approximation. In all cases, we study in detail the convergence behavior of the renormalization as the q-point sampling goes to infinity and the imaginary broadening parameter goes to zero. This allows extrapolation, thus enabling a systematic way to converge the renormalization for both polar and non-polar materials. Finally, the adiabatic and non-adiabatic theory, with corrections for the divergence problem, are applied to the study of five semiconductors and insulators: a-AlN, b-AlN, BN, diamond and silicon. For these five materials, we present the zero-point renormalization, temperature dependence, phonon-induced lifetime broadening and the renormalized electronic bandstructure.Comment: 27 pages and 26 figure

The Distribution, Excitation and Formation of Cometary Molecules: Methanol, Methyl Cyanide and Ethylene Glycol

We present an interferometric and single dish study of small organic species toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105+/-5 K and an average production rate ratio Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis observations of CH3OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of 200+/-10 K and a production rate ratio of Q(CH3CN)/Q(H2O)~0.017% at ~1 AU. The non-detection of a previously claimed transition of cometary (CH2OH)2 toward Comet Hale-Bopp with the 12m telescope indicates a compact distribution of emission, D<9'' (<8500 km). For the single-dish observations of Comet T7 LINEAR, we find an excitation temperature of CH3OH of 35+/-5 K and a CH3OH production rate ratio of Q(CH3OH)/Q(H2O)~1.5% at ~0.3 AU. Our data support current chemical models that CH3OH, CH3CN and (CH2OH)2 are parent nuclear species distributed into the coma via direct sublimation off cometary ices from the nucleus with no evidence of significant production in the outer coma.Comment: accepted for publication in Ap

Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications

We present a simple implementation of the dynamical mean-field theory approach to the electronic structure of strongly correlated materials. This implementation achieves full self-consistency over the charge density, taking into account correlation-induced changes to the total charge density and effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used, and the charge density is computed from moments of the many body momentum-distribution matrix. The calculation of the total energy is also considered, with a proper treatment of high-frequency tails of the Green's function and self-energy. The method is illustrated on two materials with well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the gamma-phase of metallic cerium, using the Hubbard-I approximation to the dynamical mean-field self-energy. The momentum-integrated spectral function and momentum-resolved dispersion of the Hubbard bands are calculated, as well as the volume-dependence of the total energy. We show that full self-consistency over the charge density, taking into account its modification by strong correlations, can be important for the computation of both thermodynamical and spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B

Phonon dispersion and electron-phonon coupling in MgB_2 and AlB_2

We present a first principles investigation of the lattice dynamics and electron-phonon coupling of the superconductor MgB_2 and the isostructural AlB_2 within the framework of density functional perturbation theory using a mixed-basis pseudopotential method. Complete phonon dispersion curves and Eliashberg functions \alpha^2F are calculated for both systems. We also report on Raman measurements, which support the theoretical findings. The calculated generalized density-of-states for MgB_2 is in excellent agreement with recent neutron-scattering experiments. The main differences in the calculated phonon spectra and \alpha^2F are related to high frequency in-plane boron vibrations. As compared to AlB_2, they are strongly softened in MgB_2 and exhibit an exceptionally strong coupling to electronic states at the Fermi energy. The total coupling constants are \lambda_{MgB_2}=0.73 and \lambda_{AlB_2}=0.43. Implications for the superconducting transition temperature are briefly discussed.Comment: 10 pages, 4 figures, to appear in Phys. Rev. Let

Correlation effects in total energy of transition metals and related properties

We present an accurate implementation of total energy calculations into the local density approximation plus dynamical mean-field theory (LDA+DMFT) method. The electronic structure problem is solved through the full potential linear Muffin-Tin Orbital (FP-LMTO) and Korringa-Kohn-Rostoker (FP-KKR) methods with a perturbative solver for the effective impurity suitable for moderately correlated systems. We have tested the method in detail for the case of Ni and investigated the sensitivity of the results to the computational scheme and to the complete self-consistency. It is demonstrated that the LDA+DMFT method can resolve a long-standing controversy between the LDA/GGA density functional approach and experiment for equilibrium lattice constant and bulk modulus of Mn.Comment: 14 pages, 5 figure