31,709 research outputs found
The star formation history of damped Lyman alpha absorbers
The local power law relationship between the surface densities of neutral
hydrogen gas and star formation rate (SFR) can be used to explore the SFR
properties of damped Lyman alpha (DLA) systems at higher redshift. We find that
while the SFR densities for DLA systems are consistent with luminous star
forming galaxies at redshifts below z~0.6, at higher redshifts their SFR
density is too low for them to provide a significant contribution to the cosmic
star formation history (SFH). This suggests that the majority of DLAs may be a
distinct population from the Lyman break galaxies (LBGs) or submillimeter
star-forming galaxies that together dominate the SFR density at high redshift.
It is also possible that the DLAs do not trace the bulk of the neutral gas at
high redshift. The metallicity properties of DLAs are consistent with this
interpretation. The DLAs show a metal mass density lower by two orders of
magnitude at all redshifts than that inferred from the SFH of the universe.
These results are consistent with DLAs being dominated by low mass systems
having low SFRs or a late onset of star formation, similar to the star
formation histories of dwarf galaxies in the local universe.Comment: 9 pages, 5 figures, accepted for publication in Ap
Two Modes of Solid State Nucleation - Ferrites, Martensites and Isothermal Transformation Curves
When a crystalline solid such as iron is cooled across a structural
transition, its final microstructure depends sensitively on the cooling rate.
For instance, an adiabatic cooling across the transition results in an
equilibrium `ferrite', while a rapid cooling gives rise to a metastable twinned
`martensite'. There exists no theoretical framework to understand the dynamics
and conditions under which both these microstructures obtain. Existing theories
of martensite dynamics describe this transformation in terms of elastic strain,
without any explanation for the occurence of the ferrite. Here we provide
evidence for the crucial role played by non-elastic variables, {\it viz.},
dynamically generated interfacial defects. A molecular dynamics (MD) simulation
of a model 2-dimensional (2d) solid-state transformation reveals two distinct
modes of nucleation depending on the temperature of quench. At high
temperatures, defects generated at the nucleation front relax quickly giving
rise to an isotropically growing `ferrite'. At low temperatures, the defects
relax extremely slowly, forcing a coordinated motion of atoms along specific
directions. This results in a twinned critical nucleus which grows rapidly at
speeds comparable to that of sound. Based on our MD results, we propose a
solid-state nucleation theory involving the elastic strain and non-elastic
defects, which successfully describes the transformation to both a ferrite and
a martensite. Our work provides useful insights on how to formulate a general
dynamics of solid state transformations.Comment: 3 pages, 4 B/W + 2 color figure
Efficiency Ranking of IT Service-Producing Firms: Case of Indian Multinationals
Production functions often study the output of physical products with capital and labor inputs. Instead, we use 2004 to 2016 data for 55 In- dian multinational companies to assess the production of services. Our estimates of flexible production functions yield estimates of scale elasticity (SCE) and elasticity of substitution (EOS) for pooled data. A subset of 31 companies with relatively complete data yields their individual SCE and EOS values, revealing their heterogeneity. Sorting the 31 companies by their SCE help name scale-efficient (high SCE) and scale inefficient (low SCE) multinationals. Similarly, a listing of 31 companies sorted by EOS allows us to name companies that are (and are not) robust to input price shocks. Using stock market data on these publicly traded companies, we report the values of three stock market criteria for top ranking companies by SCE. We also study empirical causal paths from the market criteria to EOS and SCE, suggesting that SCE and EOS do drive stock market indicators implying efficient markets. Our pooled and detailed results are relevant for government policy toward the IT sector and corporate governance issues
Jensen-Shannon divergence as a measure of distinguishability between mixed quantum states
We discuss an alternative to relative entropy as a measure of distance
between mixed quantum states. The proposed quantity is an extension to the
realm of quantum theory of the Jensen-Shannon divergence (JSD) between
probability distributions. The JSD has several interesting properties. It
arises in information theory and, unlike the Kullback-Leibler divergence, it is
symmetric, always well defined and bounded. We show that the quantum JSD (QJSD)
shares with the relative entropy most of the physically relevant properties, in
particular those required for a "good" quantum distinguishability measure. We
relate it to other known quantum distances and we suggest possible applications
in the field of the quantum information theory.Comment: 14 pages, corrected equation 1
Oxidation of Aliphatic Ketones, Substituted Acetophenones & Cyclic Ketones by Potassium Permanganate
524-52
Kinetics & Mechanism of Halogenation of Anilines, p-Toluidine & p-Nitroaniline by Chloramine-T
485-48
Theory of Hysteresis Loop in Ferromagnets
We consider three mechanisms of hysteresis phenomena in alternating magnetic
field: the domain wall motion in a random medium, the nucleation and the
retardation of magnetization due to slow (critical) fluctuations. We construct
quantitative theory for all these processes. The hysteresis is characterized by
two dynamic threshold fields, by coercive field and by the so-called reversal
field. Their ratios to the static threshold field is shown to be function of
two dimensionless variables constituted from the frequency and amplitude of the
ac field as well as from some characteristics of the magnet. The area and the
shape of the hysteresis loop are found. We consider different limiting cases in
which power dependencies are valid. Numerical simulations show the domain wall
formation and propagation and confirm the main theoretical predictions. Theory
is compared with available experimental data.Comment: RevTex, 13 pages, 8 figures (PostScript), acknowledgements adde
Shear flow induced isotropic to nematic transition in a suspension of active filaments
We study the effects of externally applied shear flow on a model of
suspensions of motors and filaments, via the equations of active hydrodynamics
[PRL {\bf 89} (2002) 058101; {\bf 92} (2004) 118101]. In the absence of shear,
the orientationally ordered phase of {\it both} polar and apolar active
particles is always unstable at zero-wavenumber. An imposed steady shear large
enough to overcome the active stresses stabilises both apolar and moving polar
phases. Our work is relevant to {\it in vitro} studies of active filaments, the
reorientation of endothelial cells subject to shear flow and shear-induced
motility of attached cells.Comment: 8 pages, 4 figures submitted to Europhysics Letter
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