449 research outputs found
Contour-time approach to the Bose-Hubbard model in the strong coupling regime: Studying two-point spatio-temporal correlations at the Hartree-Fock-Bogoliubov level
We develop a formalism that allows the study of correlations in space and
time in both the superfluid and Mott insulating phases of the Bose-Hubbard
Model. Specifically, we obtain a two particle irreducible effective action
within the contour-time formalism that allows for both equilibrium and out of
equilibrium phenomena. We derive equations of motion for both the superfluid
order parameter and two-point correlation functions. To assess the accuracy of
this formalism, we study the equilibrium solution of the equations of motion
and compare our results to existing strong coupling methods as well as exact
methods where possible. We discuss applications of this formalism to out of
equilibrium situations.Comment: 41 pages, 7 figures. arXiv admin note: substantial text overlap with
arXiv:1606.0411
Patterns of differential introgression in a salamander hybrid zone: inferences from genetic data and ecological niche modelling
Hybrid zones have yielded considerable insight into many evolutionary processes, including speciation and the maintenance of species boundaries. Presented here are analyses from a hybrid zone that occurs among three salamanders – Plethodon jordani , Plethodon metcalfi and Plethodon teyahalee – from the southern Appalachian Mountains. Using a novel statistical approach for analysis of non-clinal, multispecies hybrid zones, we examined spatial patterns of variation at four markers: single-nucleotide polymorphisms (SNPs) located in the mtDNA ND2 gene and the nuclear DNA ILF3 gene, and the morphological markers of red cheek pigmentation and white flecks. Concordance of the ILF3 marker and both morphological markers across four transects is observed. In three of the four transects, however, the pattern of mtDNA is discordant from all other markers, with a higher representation of P. metcalfi mtDNA in the northern and lower elevation localities than is expected given the ILF3 marker and morphology. To explore whether climate plays a role in the position of the hybrid zone, we created ecological niche models for P. jordani and P. metcalfi . Modelling results suggest that hybrid zone position is not determined by steep gradients in climatic suitability for either species. Instead, the hybrid zone lies in a climatically homogenous region that is broadly suitable for both P. jordani and P. metcalfi . We discuss various selective (natural selection associated with climate) and behavioural processes (sex-biased dispersal, asymmetric reproductive isolation) that might explain the discordance in the extent to which mtDNA and nuclear DNA and colour-pattern traits have moved across this hybrid zone.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/79135/1/j.1365-294X.2010.04796.x.pd
Nanoscale covariance magnetometry with diamond quantum sensors
Nitrogen vacancy (NV) centers in diamond are atom-scale defects with long
spin coherence times that can be used to sense magnetic fields with high
sensitivity and spatial resolution. Typically, the magnetic field projection at
a single point is measured by averaging many sequential measurements with a
single NV center, or the magnetic field distribution is reconstructed by taking
a spatial average over an ensemble of many NV centers. In averaging over many
single-NV center experiments, both techniques discard information. Here we
propose and implement a new sensing modality, whereby two or more NV centers
are measured simultaneously, and we extract temporal and spatial correlations
in their signals that would otherwise be inaccessible. We analytically derive
the measurable two-point correlator in the presence of environmental noise,
quantum projection noise, and readout noise. We show that optimizing the
readout noise is critical for measuring correlations, and we experimentally
demonstrate measurements of correlated applied noise using spin-to-charge
readout of two NV centers. We also implement a spectral reconstruction protocol
for disentangling local and nonlocal noise sources, and demonstrate that
independent control of two NV centers can be used to measure the temporal
structure of correlations. Our covariance magnetometry scheme has numerous
applications in studying spatiotemporal structure factors and dynamics, and
opens a new frontier in nanoscale sensing
An Observed Correlation Between Thermal and Non-Thermal Emission in Gamma-Ray Bursts
Recent observations by the Gamma-ray Space Telescope have confirmed
the existence of thermal and non-thermal components in the prompt photon
spectra of some Gamma-ray bursts (GRBs). Through an analysis of six bright
Fermi GRBs, we have discovered a correlation between the observed photospheric
and non-thermal -ray emission components of several GRBs using a
physical model that has previously been shown to be a good fit to the Fermi
data. From the spectral parameters of these fits we find that the
characteristic energies, and , of these two components are
correlated via the relation which varies from
GRB to GRB. We present an interpretation in which the value of index
indicates whether the jet is dominated by kinetic or magnetic energy. To date,
this jet composition parameter has been assumed in the modeling of GRB outflows
rather than derived from the data
Candidate X-ray-Emitting OB Stars in the Carina Nebula Identified Via Infrared Spectral Energy Distributions
We report the results of a new survey of massive, OB stars throughout the
Carina Nebula using the X-ray point source catalog provided by the Chandra
Carina Complex Project (CCCP) in conjunction with infrared (IR) photometry from
the Two Micron All-Sky Survey and the Spitzer Space Telescope Vela--Carina
survey. Mid-IR photometry is relatively unaffected by extinction, hence it
provides strong constraints on the luminosities of OB stars, assuming that
their association with the Carina Nebula, and hence their distance, is
confirmed. We fit model stellar atmospheres to the optical (UBV) and IR
spectral energy distributions (SEDs) of 182 OB stars with known spectral types
and measure the bolometric luminosity and extinction for each star. We find
that the extinction law measured toward the OB stars has two components:
Av=1--1.5 mag produced by foreground dust with a ratio of total-to-selective
absorption Rv=3.1 plus a contribution from local dust with Rv>4.0 in the Carina
molecular clouds that increases as Av increases. Using X-ray emission as a
strong indicator of association with Carina, we identify 94 candidate OB stars
with Lbol\geq10^4 Lsun by fitting their IR SEDs. If the candidate OB stars are
eventually confirmed by follow-up spectroscopic observations, the number of
cataloged OB stars in the Carina Nebula will increase by ~50%. Correcting for
incompleteness due to OB stars falling below the Lbol cutoff or the CCCP
detection limit, these results potentially double the size of the young massive
stellar population.Comment: 19 pages, 8 figures, accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers, including a version of this article with
high-quality figures, are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html (through 2011
at least
The 3rd Fermi GBM Gamma-Ray Burst Catalog: The First Six Years
Since its launch in 2008, the Fermi Gamma-ray Burst Monitor (GBM) has
triggered and located on average approximately two gamma-ray bursts (GRB) every
three days. Here we present the third of a series of catalogs of GRBs detected
by GBM, extending the second catalog by two more years, through the middle of
July 2014. The resulting list includes 1405 triggers identified as GRBs. The
intention of the GBM GRB catalog is to provide information to the community on
the most important observables of the GBM detected GRBs. For each GRB the
location and main characteristics of the prompt emission, the duration, peak
flux and fluence are derived. The latter two quantities are calculated for the
50-300~keV energy band, where the maximum energy release of GRBs in the
instrument reference system is observed, and also for a broader energy band
from 10-1000 keV, exploiting the full energy range of GBM's low-energy NaI(Tl)
detectors. Using statistical methods to assess clustering, we find that the
hardness and duration of GRBs are better fitted by a two-component model with
short-hard and long-soft bursts, than by a model with three components.
Furthermore, information is provided on the settings and modifications of the
triggering criteria and exceptional operational conditions during years five
and six in the mission. This third catalog is an official product of the Fermi
GBM science team, and the data files containing the complete results are
available from the High-Energy Astrophysics Science Archive Research Center
(HEASARC).Comment: 225 pages, 13 figures and 8 tables. Accepted for publication in
Astrophysical Journal Supplement 201
A Comparison of Copper Abundances in Globular Cluster and Halo Field Giant Stars
We derive [Cu/Fe] for 117 giant stars in ten globular clusters (M3, M4, M5,
M10, M13, M15, M71, NGC 7006, NCG 288, and NGC 362) and find that globular
cluster Cu abundances appear to follow [Cu/Fe] trends found in the field. This
result is interesting in light of recent work which indicates that the globular
cluster Omega Centauri shows no trend in [Cu/Fe] with [Fe/H] over the abundance
range -2.0 <[Fe/H]< -0.8. Of particular interest are the two clusters M4 and
M5. While at a similar metallicity ([Fe/H] ~- 1.2), they differ greatly in some
elemental abundances: M4 is largely overabundant in Si, Ba, and La compared to
M5. We find that it is also overabundant in Cu with respect to M5, though this
overabundance is in accord with [Cu/Fe] ratios found in the field.Comment: 39 pages, 7 figures, to appear in April 2003 A
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