3,216 research outputs found
Flux jumps, Second Magnetization Peak anomaly and the Peak Effect phenomenon in single crystals of and
We present magnetization measurements in single crystals of the tetragonal
compound, which exhibit the phenomenon of peak effect as well as
the second magnetization peak anomaly for H 0.5T (H c). At the lower
field (50mT H 200mT), we have observed the presence of flux jumps,
which seem to relate to a structural change in the local symmetry of the flux
line lattice (a first order re-orientation transition across a local field in
some parts of the sample, in the range of 100mT to 150mT). These flux jumps are
also observed in a single crystal of for H c in the field
region from 2 mT to 25 mT, which are compatible with the occurrence of a
re-orientation transition at a lower field in a cleaner crystal of this
compound, as compared to those of . Vortex phase diagrams drawn for
H c in and show that the ordered elastic glass
phase spans a larger part of (H, T) space in the former as compared to latter,
thereby, reaffirming the difference in the relative purity of the two samples.Comment: 11 pages, 14 figure
A tight lower bound instance for k-means++ in constant dimension
The k-means++ seeding algorithm is one of the most popular algorithms that is
used for finding the initial centers when using the k-means heuristic. The
algorithm is a simple sampling procedure and can be described as follows: Pick
the first center randomly from the given points. For , pick a point to
be the center with probability proportional to the square of the
Euclidean distance of this point to the closest previously chosen
centers.
The k-means++ seeding algorithm is not only simple and fast but also gives an
approximation in expectation as shown by Arthur and Vassilvitskii.
There are datasets on which this seeding algorithm gives an approximation
factor of in expectation. However, it is not clear from these
results if the algorithm achieves good approximation factor with reasonably
high probability (say ). Brunsch and R\"{o}glin gave a dataset where
the k-means++ seeding algorithm achieves an approximation ratio
with probability that is exponentially small in . However, this and all
other known lower-bound examples are high dimensional. So, an open problem was
to understand the behavior of the algorithm on low dimensional datasets. In
this work, we give a simple two dimensional dataset on which the seeding
algorithm achieves an approximation ratio with probability
exponentially small in . This solves open problems posed by Mahajan et al.
and by Brunsch and R\"{o}glin.Comment: To appear in TAMC 2014. arXiv admin note: text overlap with
arXiv:1306.420
A Partition-centric Distributed Algorithm for Identifying Euler Circuits in Large Graphs
Finding the Eulerian circuit in graphs is a classic problem, but inadequately
explored for parallel computation. With such cycles finding use in neuroscience
and Internet of Things for large graphs, designing a distributed algorithm for
finding the Euler circuit is important. Existing parallel algorithms are
impractical for commodity clusters and Clouds. We propose a novel
partition-centric algorithm to find the Euler circuit, over large graphs
partitioned across distributed machines and executed iteratively using a Bulk
Synchronous Parallel (BSP) model. The algorithm finds partial paths and cycles
within each partition, and refines these into longer paths by recursively
merging the partitions. We describe the algorithm, analyze its complexity,
validate it on Apache Spark for large graphs, and offer experimental results.
We also identify memory bottlenecks in the algorithm and propose an enhanced
design to address it.Comment: To appear in Proceedings of 5th IEEE International Workshop on
High-Performance Big Data, Deep Learning, and Cloud Computing, In conjunction
with The 33rd IEEE International Parallel and Distributed Processing
Symposium (IPDPS 2019), Rio de Janeiro, Brazil, May 20th, 201
Towards decoupling the effects of permeability and roughness on turbulent boundary layers
Boundary layer flow over a realistic porous wall might contain both the
effects of wall-permeability and wall-roughness. These two effects are
typically examined in the context of a rough-wall flow, i.e., by defining a
``roughness'' length or equivalent to capture the effect of the surface on
momentum deficit/drag. In this work, we examine the hypothesis of Esteban et
al. (2022), that a turbulent boundary layer over a porous wall could be
modelled as a superposition of the roughness effects on the permeability
effects by using independently obtained information on permeability and
roughness. We carry out wind tunnel experiments at high Reynolds number () on various combinations of porous walls where
different roughnesses are overlaid over a given permeable wall. Measurements
are also conducted on the permeable wall as well as the rough walls
independently to obtain the corresponding lengthscales. Analysis of mean flow
data across all these measurements suggests that an empirical formulation can
be obtained where the momentum deficit () is modelled as a
combination of independently obtained roughness and permeability lengthscales.
This formulation assumes the presence of outer-layer similarity across these
different surfaces, which is shown to be valid at high Reynolds numbers.
Finally, this decoupling approach is equivalent to the area-weighted power-mean
of the respective permeability and roughness lengthscales, consistent with the
approach recently suggested by Hutchins et al. (2023) to capture the effects of
heterogeneous rough surfaces.Comment: Under review for publication in JFM Rapid
Scattering Transparency of Clouds in Exoplanet Transit Spectra
The presence of aerosols in an exoplanet atmosphere can veil the underlying
material and can lead to a flat transmission spectrum during primary transit
observations. In this work, we explore forward scattering effects from
super-micron sized aerosol particles present in the atmosphere of a transiting
exoplanet. We find that the impacts of forward scattering from larger aerosols
can significantly impact exoplanet transits and the strength of these effects
can be dependent on wavelength. In certain cloud configurations, the
forward-scattered light can effectively pass through the clouds unhindered,
thus rendering the clouds transparent. The dependence of the aerosol scattering
properties on wavelength can then lead to a positive slope in the transit
spectrum. These slopes are characteristically different from both Rayleigh and
aerosol absorption slopes. As examples, we demonstrate scattering effects for
both a rocky world and a hot Jupiter. In these models, the predicted spectral
slopes due to forward scattering effects can manifest in the transit spectrum
at the level of 10s to 100s of parts per million and, hence, could
be observable with NASA's James Webb Space Telescope.Comment: 9 pages, 7 figures, published in MNRA
Optical transitions and nature of Stokes shift in spherical CdS quantum dots
We study the structure of the energy spectra along with the character of the
states participating in optical transitions in colloidal CdS quantum dots (QDs)
using the {\sl ab initio} accuracy charge patching method combined with the
%pseudopotential based folded spectrum calculations of electronic structure of
thousand-atom nanostructures. In particular, attention is paid to the nature of
the large resonant Stokes shift observed in CdS quantum dots. We find that the
top of the valence band state is bright, in contrast with the results of
numerous {\bf kp} calculations, and determine the limits of
applicability of the {\bf kp} approach. The calculated electron-hole
exchange splitting suggests the spin-forbidden valence state may explain the
nature of the ``dark exciton'' in CdS quantum dots.Comment: 5 pages, 4 figure
Mechanism of Cyanide Toxicity and Efficacy of its Antidotes
This paper attempts to review the various antidotes available for countering cyanide threat in the light of the toxicity associated with it. It also critically evaluates the drawbacks and advantages of these antidotes for their therapeutic and/or prophylactic utility. The physico-chemical properties of hydrogen cyanide which make it a chemical warfare agent have also been highlighted. In an attempt to make the complex chemical and biological processes understandable, the chemical structures of the antidotes have been included and simple mechanistic pathways have been used to show the role of antidotes in activating the inhibited enzymes
An Overview of Charge Pump for Phase Lock Loop System for High Frequency Application.
Phase lock loop is fundamental buliding block of modern communication system. Phase lock loop are typically used to provide local oscillator function in radio reciver or transmitter. The design methodology and test result of charge pump structure for phase lock loop application are presented. The structure is composed to two charge/ discharge block. This paper provides study of various charge pump and discuss the technology that is used to design charge pump
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