53,356 research outputs found
The Tail of the HI Mass Function
The contribution of extragalactic objects with HI masses below
to the HI mass function remains uncertain. Several aspects of the detection of
low-mass sources in HI surveys are not always considered, and as a result
different analysis techniques yield widely different estimates for their number
density. It is suggested at one extreme that the number density of galaxies
follows a shallow Schechter power-law slope, and at the other extreme that it
follows a steep faint-end rise like that found for field optical sources. Here
we examine a variety of selection effects, issues of completeness, and
consequences of LSS. We derive results for the large Arecibo Dual Beam Survey
which indicate that the field mass function does rise steeply, while within the
Virgo Cluster environs, the slope appears to be much shallower. Dependence on
the local density of galaxies may partially explain differences between
surveys.Comment: 8 pages, presented at Mapping the Hidden Universe: The Universe in
HI. eds Kraan-Korteweg, Henning, Andernac
Formation of nanocrystals based on decomposition in the amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy
Primary crystallization and decomposition in the bulk amorphous alloy Ar41.2Ti13.8Cu12.5Ni10Be22.5 have been studied by small angle neutron scattering (SANS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at q=0.46 nm(^-1) after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals
Decomposition and primary crystallization in undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 melts
Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glasses were prepared by cooling the melt with a rate of about 10 K/s and investigated with respect to their chemical and structural homogeneity by atom probe field ion microscopy and transmission electron microscopy. The measurements on these slowly cooled samples reveal that the alloy exhibits phase separation in the undercooled liquid state. Significant composition fluctuations are found in the Be and Zr concentration but not in the Ti, Cu, and Ni concentration. The decomposed microstructure is compared with the microstructure obtained upon primary crystallization, suggesting that the nucleation during primary crystallization of this bulk glass former is triggered by the preceding diffusion controlled decomposition in the undercooled liquid state
Gas and Stars in an HI-Selected Galaxy Sample
We present the results of a J-band study of the HI-selected Arecibo Dual-Beam
Survey and Arecibo Slice Survey galaxy samples using the 2 Micron All-Sky
Survey data. We find that these galaxies span a wide range of stellar and gas
properties. However, despite the diversity within the samples, we find a very
tight correlation between luminosity and size in the J-band, similar to that we
previously found (Rosenberg & Schneider 2003) between the HI mass and size. We
also find that the correlation between the baryonic mass and the J-band
diameter is even tighter than between the baryonic mass and the rotational
velocity.Comment: AJ in press, 17 pages (including tables and figures) + 6 additional
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Magnetic field induced finite size effect in type-II superconductors
We explore the occurrence of a magnetic field induced finite size effect on
the specific heat and correlation lengths of anisotropic type-II
superconductors near the zero field transition temperature Tc. Since near the
zero field transition thermal fluctuations are expected to dominate and with
increasing field strength these fluctuations become one dimensional, whereupon
the effect of fluctuations increases, it appears unavoidable to account for
thermal fluctuations. Invoking the scaling theory of critical phenomena it is
shown that the specific heat data of nearly optimally doped YBa2Cu3O7-x are
inconsistent with the traditional mean-field and lowest Landau level
predictions of a continuous superconductor to normal state transition along an
upper critical field Hc2(T). On the contrary, we observe agreement with a
magnetic field induced finite size effect, whereupon even the correlation
length longitudinal to the applied field H cannot grow beyond the limiting
magnetic length L(H). It arises because with increasing magnetic field the
density of vortex lines becomes greater, but this cannot continue indefinitely.
L(H) is then roughly set on the proximity of vortex lines by the overlapping of
their cores. Thus, the shift and the rounding of the specific heat peak in an
applied field is traced back to a magnetic field induced finite size effect in
the correlation length longitudinal to the applied field.Comment: 8 pages, 4 figure
Optimization of -Layer Systems for Josephson Junctions from a Microstructure Point of View
-layer systems are frequently used for Josephson junction-based
superconducting devices. Although much work has been devoted to the
optimization of the superconducting properties of these devices, systematic
studies on influence of deposition conditions combined with structural analyses
on the nanoscale are rare up to now. We have focused on the optimization of the
structural properties of -layer systems deposited on Si(111)
substrates with a particular focus on the thickness homogeneity of the
-tunnel barrier. A standard high-vacuum electron-beam deposition system
was used and the effect of substrate pretreatment, different Al-deposition
temperatures and Al-deposition rates was studied. Transmission electron
microscopy was applied to analyze the structural properties of the
-layer systems to determine the thickness homogeneity of the
layer, grain size distribution in the Al layers, Al-grain boundary
types and the morphology of the interface. We show that the
structural properties of the lower Al layer are decisive for the structural
quality of the whole -layer system. Optimum conditions yield an
epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness
variation of only 1.6 nm over more than 10 and large lateral grain
sizes up to 1 . Thickness fluctuations of the -tunnel barrier are
minimized on such an Al layer which is essential for the homogeneity of the
tunnel current. Systematic variation of the Al-deposition rate and deposition
temperature allows to develop an understanding of the growth mechanisms
A Comparison of Phycocyanins from Three Different Species of Cyanobacteria Employing Resonance-Enhanced Coherent Anti-Stokes Raman Spectroscopy
Resonance-enhanced coherent anti-Stokes Raman spectra are recorded for monomers and trimers of phycocyanin from three different cyanobacteria: Westiellopsis prolifica, Mastigocladus laminosus and Spirulina platensis. It is shown that upon aggregation from monomer to trimer the electronic structures of both the α84 and β84 chromophores are changed. The spectra of the trimers originating from S. platensis and M. laminosus are very similar to each other, but distinctly different from the spectrum of W. prolifica
Crystallization of Carbon Oxygen Mixtures in White Dwarf Stars
We determine the phase diagram for dense carbon/ oxygen mixtures in White
Dwarf (WD) star interiors using molecular dynamics simulations involving liquid
and solid phases. Our phase diagram agrees well with predictions from Ogata et
al. and Medin and Cumming and gives lower melting temperatures than Segretain
et al. Observations of WD crystallization in the globular cluster NGC 6397 by
Winget et al. suggest that the melting temperature of WD cores is close to that
for pure carbon. If this is true, our phase diagram implies that the central
oxygen abundance in these stars is less than about 60%. This constraint, along
with assumptions about convection in stellar evolution models, limits the
effective S factor for the C()O reaction to
S_{300} <= 170 keV barns.Comment: 4 pages, 2 figures, Phys. Rev. Lett. in pres
Small-angle x-ray-scattering study of phase separation and crystallization in the bulk amorphous Mg62Cu25Y10Li3 alloy
We report on a small-angle x-ray-scattering (SAXS) and differential scanning calorimetry study of phase separation and crystallization in rapidly quenched amorphous Mg62Cu25Y10Li3 alloy samples. Differential scanning calorimetry demonstrates the occurrence of crystallization and grain growth upon isothermal annealing of these samples at 135 °C. The SAXS studies show the presence of large inhomogeneities even in the rapidly quenched as-prepared Mg62Cu25Y10Li3 alloy that is attributed to phase separation in the undercooled liquid during the cooling process. After isothermal annealing at 135 °C for longer than 30 min the samples exhibit a strong SAXS intensity that monotonically increases with increasing annealing time. During heat treatment, crystallization and growth of a nanocrystalline bcc-Mg7Li3 phase occurs in the Y-poor and MgLi-rich domains. The initially rough boundaries of the nanocrystals become sharper with increasing annealing time. Anomalous small-angle x-ray-scattering investigations near the Cu K edge indicate that while Cu is distributed homogeneously in the as-prepared sample, a Cu composition gradient develops between the matrix and the bcc-Mg7Li3 nanocrystals in the annealed sample
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