27,328 research outputs found
Correlation of Low z Lyman-alpha Absorbers with HI-selected Galaxies
In this work, observational evidence for the connection between low column
density Lyman-alpha absorbers and large-scale structure traced by gas-rich
galaxies is investigated. The HI Parkes All Sky Survey (HIPASS) galaxy
catalogue is cross-correlated with known low redshift, low column density
Lyman-alpha absorbers from the literature. The absorber-galaxy
cross-correlation function shows that on scales from 1 to 10 h^-1 Mpc,
absorbers are imbedded in halos with masses similar to that of galaxy groups.
This statistical evidence suggests that galaxy groups could be the dominant
environment of low column density Lyman-alpha absorbers at z=0.Comment: 3 pages, 1 figure, to appear in IAU 199 conf. proc.: "Probing
Galaxies through Quasar Absorption Lines," eds. Williams, Shu, Menar
Cross correlation of Lyman-alpha absorbers with gas-rich galaxies
The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated
with known low redshift, low column density (N_HI <10^15 cm^-2) Lyman-alpha
absorbers from the literature. The redshift-space correlation is found to be
similar in strength to HIPASS galaxy self-clustering (correlation length
s_0,ag=6+/-4 and s_0,gg=3.1+/-0.5 h^-1 Mpc respectively). In real-space the
cross-correlation is stronger than the galaxy auto-correlation (correlation
length r_0,ag=7.2+/-1.4 and r_0,gg=3.5+/-0.7 h^-1 Mpc respectively) on scales
from 1-10 h^-1 Mpc, ruling out the mini-halo model for the confinement
Lyman-alpha absorbers at the 99 percent confidence level. Provided that the
cause of the strong cross-correlation is purely gravitational, the ratio of
correlation lengths suggest that absorbers are embedded in dark matter haloes
with masses log(M/Msun)=14.2 h^-1, similar to those of galaxy groups. The
flattening of the cross-correlation at separations less than ~600 h^-1 kpc
could correspond to the thickness of filaments in which absorbers are embedded.
This work provides indirect statistical evidence for the notion that galaxy
groups and large-scale filaments, particularly those that comprise gas-rich
galaxies, are the dominant environments of low column density Lyman-alpha
absorbers at z=0.Comment: 10 pages, 10 figures, accepted for publication in MNRAS, minor typos
fixed and references update
Crystalline topological phases as defect networks
A crystalline topological phase is a topological phase with spatial
symmetries. In this work, we give a very general physical picture of such
phases: a topological phase with spatial symmetry (with internal symmetry
) is described by a *defect network*: a -symmetric
network of defects in a topological phase with internal symmetry
. The defect network picture works both for
symmetry-protected topological (SPT) and symmetry-enriched topological (SET)
phases, in systems of either bosons or fermions. We derive this picture both by
physical arguments, and by a mathematical derivation from the general framework
of [Thorngren and Else, Phys. Rev. X 8, 011040 (2018)]. In the case of
crystalline SPT phases, the defect network picture reduces to a previously
studied dimensional reduction picture, thus establishing the equivalence of
this picture with the general framework of Thorngren and Else applied to
crystalline SPTs.Comment: 13 pages + 2 pages of appendices. v3 published version, with better
justification of the equivalence relatio
Entanglement entropy of composite Fermi liquid states on the lattice: In support of the Widom formula
Quantum phases characterized by surfaces of gapless excitations are known to
violate the otherwise ubiquitous boundary law of entanglement entropy in the
form of a multiplicative log correction: . Using
variational Monte Carlo, we calculate the second R\'enyi entropy for a model
wavefunction of the composite Fermi liquid (CFL) state defined on the
two-dimensional triangular lattice. By carefully studying the scaling of the
total R\'enyi entropy and, crucially, its contributions from the modulus and
sign of the wavefunction on various finite-size geometries, we argue that the
prefactor of the leading term is equivalent to that in the analogous
free fermion wavefunction. In contrast to the recent results of Shao et al.
[PRL 114, 206402 (2015)], we thus conclude that the "Widom formula" holds even
in this non-Fermi liquid CFL state. More generally, our results further
elucidate---and place on a more quantitative footing---the relationship between
nontrivial wavefunction sign structure and entanglement
scaling in such highly entangled gapless phases.Comment: 8 pages, 6 figure
Evolutionary approach for finding the atomic structure of steps on stable crystal surfaces
The problem addressed here can be concisely formulated as follows: Given a stable surface orientation with a known reconstruction and given a direction in the plane of this surface, find the atomic structure of the steps oriented along that direction. We report a robust and generally applicable variable-number genetic algorithm for determining the atomic configuration of crystallographic steps, and exemplify it by finding structures for several types of monatomic steps on Si(114)-2×1. We show that the location of the step edge with respect to the terrace reconstructions, the step width (number of atoms), and the positions of the atoms in the step region can all be simultaneously determined
Systems design analysis applied to launch vehicle configuration
As emphasis shifts from optimum-performance aerospace systems to least lift-cycle costs, systems designs must seek, adapt, and innovate cost improvement techniques in design through operations. The systems design process of concept, definition, and design was assessed for the types and flow of total quality management techniques that may be applicable in a launch vehicle systems design analysis. Techniques discussed are task ordering, quality leverage, concurrent engineering, Pareto's principle, robustness, quality function deployment, criteria, and others. These cost oriented techniques are as applicable to aerospace systems design analysis as to any large commercial system
The stability of strained H:Si(105) and H:Ge(105) surfaces
We report atomic scale studies of the effect of applied strain and hydrogen
environment on the reconstructions of the (105) Si and Ge surfaces. Surface
energy calculations for monohydride-terminated (001) and (105) reconstructions
reveal that the recently established single-height rebonded model is unstable
not only with respect to (001), but also in comparison to other monohydride
(105) structures. This finding persists for both Si and Ge, for applied biaxial
strains from -4% to 4%, and for nearly the entire relevant domain of the
chemical potential of hydrogen, thus providing an explanation for the recently
observed H-induced destabilization of the Ge(105) surface
A spectral difference between silicates in Comet Halley and interstellar silicates
The authors obtained an intermediate resolution (1 percent) spectrum of the 8 to 13 micron region in Comet Halley which shows a prominent silicate emission feature with structure not observed before in other comets or in interstellar silicates. They confirm the presence of a strong 11.3 micron peak reported by Bregman et al. (1987) and find evidence for additional structure in the band. The 11.3 micron peak represents the main difference between the Halley Spectrum and that of Comet Kohoutek. The Kohoutek Spectrum is similar to that of the circumstellar shell around mu Ceph. Based on a comparison with the spectra of Interplanetary Dust Particles (Sandford and Walker 1985), most of which are believed to be of cometary origin, the authors attribute the 11.3 micron peak to small crystalline olivine particles, although other minerals cannot be ruled out. Their interpretation is supported by the airborne observation of four emission peaks near 24, 28, 35 and 45 microns which can also be matched with iron-magnesium silicates including crystalline olivine. Other types of silicates (such as hydrated or amorphous) are necessary to explain the width and the 9.7 micron peak of the emission observed in Comet Halley
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