4,480 research outputs found
Strongly enhanced photon collection from diamond defect centres under micro-fabricated integrated solid immersion lenses
The efficiency of collecting photons from optically active defect centres in
bulk diamond is greatly reduced by refraction and reflection at the diamond-air
interface. We report on the fabrication and measurement of a geometrical
solution to the problem; integrated solid immersion lenses (SILs) etched
directly into the surface of diamond. An increase of a factor of 10 was
observed in the saturated count-rate from a single negatively charged
nitrogen-vacancy (NV-) within a 5um diameter SIL compared with NV-s under a
planar surface in the same crystal. A factor of 3 reduction in background
emission was also observed due to the reduced excitation volume with a SIL
present. Such a system is potentially scalable and easily adaptable to other
defect centres in bulk diamond.Comment: 5 Pages, 5 figures (4 subfigures) - corrected typ
Estimating the phylogeny and divergence times of primates using a supermatrix approach
<p>Abstract</p> <p>Background</p> <p>The primates are among the most broadly studied mammalian orders, with the published literature containing extensive analyses of their behavior, physiology, genetics and ecology. The importance of this group in medical and biological research is well appreciated, and explains the numerous molecular phylogenies that have been proposed for most primate families and genera. Composite estimates for the entire order have been infrequently attempted, with the last phylogenetic reconstruction spanning the full range of primate evolutionary relationships having been conducted over a decade ago.</p> <p>Results</p> <p>To estimate the structure and tempo of primate evolutionary history, we employed Bayesian phylogenetic methods to analyze data supermatrices comprising 7 mitochondrial genes (6,138 nucleotides) from 219 species across 67 genera and 3 nuclear genes (2,157 nucleotides) from 26 genera. Many taxa were only partially represented, with an average of 3.95 and 5.43 mitochondrial genes per species and per genus, respectively, and 2.23 nuclear genes per genus. Our analyses of mitochondrial DNA place Tarsiiformes as the sister group of Strepsirrhini. Within Haplorrhini, we find support for the primary divergence of Pitheciidae in Platyrrhini, and our results suggest a sister grouping of African and non-African colobines within Colobinae and of Cercopithecini and Papionini within Cercopthecinae. Date estimates for nodes within each family and genus are presented, with estimates for key splits including: Strepsirrhini-Haplorrhini 64 million years ago (MYA), Lemuriformes-Lorisiformes 52 MYA, Platyrrhini-Catarrhini 43 MYA and Cercopithecoidea-Hominoidea 29 MYA.</p> <p>Conclusion</p> <p>We present an up-to-date, comprehensive estimate of the structure and tempo of primate evolutionary history. Although considerable gaps remain in our knowledge of the primate phylogeny, increased data sampling, particularly from nuclear loci, will be able to provide further resolution.</p
Using improved power electronics modeling and turbine control to improve wind turbine reliability
Improving offshore wind turbine reliability is a key industry goal to improve the availability of this renewable energy generation source. The semiconductor devices in the wind turbine power converter are traditionally considered as the most sensitive and important components to achieve this and managing their thermomechanical stressing is vital, since this is one of their principal long-term aging mechanisms. Conventional deterministic reliability prediction methods used in industrial applications are not suitable for wind turbine applications, due to the stochastic nature of the wind speed. This paper develops an electrothermal model of the power devices, which is integrated with a wind turbine system model for the investigation of power converter thermal cycling under various operating conditions. The model has been developed to eliminate the problems of pulse width modulation switching, substantially reducing simulation time. The model is used to improve the current controller tuning method to reduce thermal stresses suffered by the converter during a grid fault. The model is finally used to design a control method to alleviate a key problem of the doubly fed induction generator—severe thermal cycling caused during operation near synchronous speed
Feast and Famine: Regulation of Black Hole Growth in Low Redshift Galaxies
We analyze the observed distribution of Eddington ratios as a function of
supermassive black hole mass for a large sample of nearby galaxies drawn from
the Sloan Digital Sky Survey. We demonstrate that there are two distinct
regimes of black hole growth in nearby galaxies. The first is associated with
galaxies with significant star formation in their central kiloparsec regions,
and is characterized by a broad log-normal distribution of accretion rates
peaked at about one percent of the Eddington limit. In this regime, the
Eddington ratio distribution is independent of the mass of the black hole and
shows no further dependence on the central stellar population of the galaxy.
The second regime is associated with galaxies with old central stellar
populations, and is characterized by a power-law distribution function of
Eddington ratios. In this regime, the time-averaged mass accretion rate onto
black holes is proportional to the mass of stars in the galaxy bulge, with a
constant of proportionality that depends on the mean stellar age of the stars.
This result is once again independent of black hole mass. We show that both the
slope of the power-law and the decrease in the accretion rate onto black holes
in old galaxies are consistent with population synthesis model predictions of
the decline in stellar mass loss rates as a function of mean stellar age. Our
results lead to a very simple picture of black hole growth in the local
Universe. If the supply of cold gas in a galaxy bulge is plentiful, the black
hole regulates its own growth at a rate that does not further depend on the
properties of the interstellar medium. Once the gas runs out, black hole growth
is regulated by the rate at which evolved stars lose their mass.Comment: 22 pages, 11 figures, accepted by MNRAS (revised version incorporates
an improved correction for star formation contribution to L[OIII]
Revealing the maternal demographic history of Panthera leo using ancient DNA and a spatially explicit genealogical analysis
Background: Understanding the demographic history of a population is critical to conservation and to our broader understanding of evolutionary processes. For many tropical large mammals, however, this aim is confounded by the absence of fossil material and by the misleading signal obtained from genetic data of recently fragmented and isolated populations. This is particularly true for the lion which as a consequence of millennia of human persecution, has large gaps in its natural distribution and several recently extinct populations. Results: We sequenced mitochondrial DNA from museum-preserved individuals, including the extinct Barbary lion (Panthera leo leo) and Iranian lion (P. l. persica), as well as lions from West and Central Africa. We added these to a broader sample of lion sequences, resulting in a data set spanning the historical range of lions. Our Bayesian phylogeographical analyses provide evidence for highly supported, reciprocally monophyletic lion clades. Using a molecular clock, we estimated that recent lion lineages began to diverge in the Late Pleistocene. Expanding equatorial rainforest probably separated lions in South and East Africa from other populations. West African lions then expanded into Central Africa during periods of rainforest contraction. Lastly, we found evidence of two separate incursions into Asia from North Africa, first into India and later into the Middle East. Conclusions: We have identified deep, well-supported splits within the mitochondrial phylogeny of African lions, arguing for recognition of some regional populations as worthy of independent conservation. More morphological and nuclear DNA data are now needed to test these subdivisions.European Union�s Seventh Framework Programme for research, technological development and demonstration under grant agreement no. FP7-PEOPLE-2011-IEF-298820.Scopu
The AGN fraction - velocity dispersion relation in clusters of galaxies
Some previous investigations have found that the fraction (f_AGN) of active
galactic nuclei (AGNs) is lower in clusters than in the field. This can result
from the suppression of galaxy-galaxy mergers in high-velocity dispersion
(sigma_v) clusters, if the formation and/or fueling of AGNs is directly related
to the merging process. We investigate the existence of a relation between
f_AGN and sigma_v in galaxy clusters in order to shed light on the formation
and evolution processes of AGNs and cluster galaxies. Using data from the Sloan
Digital Sky Survey we determine f_AGN and sigma_v for the clusters in two
samples, extracted from the catalogs of Popesso et al. (2006a) and Miller et
al. (2005), and excluding clusters with significant evidence for substructures.
We find a significant f_AGN-sigma_v anti-correlation. Clusters with sigma_v
lower and, respectively, higher than 500 km/s have AGN fractions of and , on average. The f_AGN-sigma_v relation can be
described by a model that assumes f_AGN is proportional to the galaxies merging
rate, plus a constant. Since f_AGN increases with decreasing sigma_v, AGNs are
likely to have played a significant role in heating the intra-cluster medium
and driving galaxy evolution in cluster precursors and groups.Comment: 4 pages, 3 figures, A&A Letter accepted for publicatio
Nuclear activity in galaxy pairs: a spectroscopic analysis of 48 UZC-BGPs
Galaxy pairs are ideal sites in which to investigate the role of interaction
on nuclear activity. For this reason we have undertaken a spectroscopic survey
of a large homogeneous sample of galaxy pairs (UZC-BGP) and we present the
results of the nuclear spectral classification of 48 pairs (more than half of
the whole sample). The fraction of emission line galaxies is extremely large,
especially among spirals (84 % and 95 %, for early and late spirals
respectively). SB is the most frequent type of nuclear activity encountered (30
% of galaxies) while AGNs are only 19%. The fractions raise to 45 % and 22 %
when considering only spirals. Late spirals are characterized by both an
unusual increase (35 %) of AGN activity and high luminosity (44 % have M_B
<-20.0 + 5log h). LLAGNs are only 8% of the total number of galaxies, but this
activity could be present in another 10 % of the galaxies (LLAGN candidates).
Absorption line galaxies reside mostly (61 %) in S0 galaxies and display the
lowest B luminosity in the sample, only 18 % of them have M_B < -20 + 5 log h,
but together with LLAGNs they are the most massive galaxies in the sample.
Intense-SB nuclei are found in galaxy pairs with galaxy-galaxy projected
separations up to 160 h^{-1} kpc suggesting that in bright isolated galaxy
pairs interaction may be at work and effective up to that distance. AGNs are
characterized by an advanced morphology while SB phenomenon occurs with the
same frequency in early and late spirals. LLAGNs and LLAGN candidates do not
always show similar properties, a finding which might confirm the heterogeneous
nature of this class of objects. Half LLAGNs are hosted in galaxies showing
visible signs of interaction with fainter companions, suggesting that minor
interactions might be a driving mechanism for a relevant fraction of LLAGNs.Comment: 19 pages, 11 figures, accepted by A&
Multicolor photometry of ten Seyfert 1 galaxies
We present BVI photometry of ten Seyfert 1 galaxies and narrow band H-alpha
images for six of these objects as well. The results indicate that the
luminosity sample distribution has an amplitude of almost 4 magnitudes with an
average of M_B=-20.7. The observed morphologies are confined to early type
galaxies. A barred structure is found in only 2 objects. Despite that early
morphological types are dominant in this sample, integrated (B-V) colors are
very blue. For instance, the SO galaxies show, on average, a (B-V)=0.78. This
effect seems to be caused by the luminosity contribution of the active nucleus
and/or the disk to the total luminosity of the galaxy. In the B band, the
contribution of the active galactic nucleus to the total luminosity of the
galaxy varies from 3% to almost 60% and the bulge to disk luminosity ratio
(L_bulge/L_disk) ranges from 0.6 to 22. Signs of tidal interactions seems to be
a common characteristic since they are observed in 6 of the objects and one of
them seems to be located in a poor cluster not yet identified in the
literature. H_alpha extended emission is rare, with only 1 galaxy showing clear
evidence of it. Luminosity profile decomposition shows that the model Gauss +
bulge + disk properly reproduces the surface brightness of the galaxies.
However, in order to account for the luminosity profile, most of the disk
galaxies needs the inner truncated exponential form with a central cutoff
radius ranging from 3 to 10 kpc. This is interpreted in terms of reddened
regions that are well identified in the B-V color maps. These regions present
very similar colors among them, with (B-V)~1.2. This fact could be associated
to the presence of dust confined in the inner regions of the galaxies.Comment: 14 pages, 25 figures. Accepted to Astronomy & Astrophysic
An adaptive quantum approximate optimization algorithm for solving combinatorial problems on a quantum computer
The quantum approximate optimization algorithm (QAOA) is a hybrid variational
quantum-classical algorithm that solves combinatorial optimization problems.
While there is evidence suggesting that the fixed form of the original QAOA
ansatz is not optimal, there is no systematic approach for finding better
ans\"atze. We address this problem by developing an iterative version of QAOA
that is problem-tailored, and which can also be adapted to specific hardware
constraints. We simulate the algorithm on a class of Max-Cut graph problems and
show that it converges much faster than the original QAOA, while simultaneously
reducing the required number of CNOT gates and optimization parameters. We
provide evidence that this speedup is connected to the concept of shortcuts to
adiabaticity.Comment: 5 pages, 3 figure
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