3,841 research outputs found
Best network chirplet-chain: Near-optimal coherent detection of unmodeled gravitation wave chirps with a network of detectors
The searches of impulsive gravitational waves (GW) in the data of the
ground-based interferometers focus essentially on two types of waveforms: short
unmodeled bursts and chirps from inspiralling compact binaries. There is room
for other types of searches based on different models. Our objective is to fill
this gap. More specifically, we are interested in GW chirps with an arbitrary
phase/frequency vs. time evolution. These unmodeled GW chirps may be considered
as the generic signature of orbiting/spinning sources. We expect quasi-periodic
nature of the waveform to be preserved independent of the physics which governs
the source motion. Several methods have been introduced to address the
detection of unmodeled chirps using the data of a single detector. Those
include the best chirplet chain (BCC) algorithm introduced by the authors. In
the next years, several detectors will be in operation. The joint coherent
analysis of GW by multiple detectors can improve the sight horizon, the
estimation of the source location and the wave polarization angles. Here, we
extend the BCC search to the multiple detector case. The method amounts to
searching for salient paths in the combined time-frequency representation of
two synthetic streams. The latter are time-series which combine the data from
each detector linearly in such a way that all the GW signatures received are
added constructively. We give a proof of principle for the full sky blind
search in a simplified situation which shows that the joint estimation of the
source sky location and chirp frequency is possible.Comment: 22 pages, revtex4, 6 figure
Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides
because the Co sits on a triangular lattice and its valence can be tuned over a
wide range by varying the Na concentration x. Up to now detailed modeling of
the rich phenomenology (which ranges from unconventional superconductivity to
enhanced thermopower) has been hampered by the difficulty of controlling pure
phases. We discovered that certain Na concentrations are specially stable and
are associated with superlattice ordering of the Na clusters. This leads
naturally to a picture of co-existence of localized spins and itinerant charge
carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our
picture brings coherence to a variety of measurements ranging from NMR to
optical to thermal transport. Our results also allow us to take the first step
towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We
suggest the local moments may form a quantum spin liquid state and we propose
experimental test of our hypothesis.Comment: 16 pages, 5 figure
Within-Subject Variability of Interferon-g Assay Results for Tuberculosis and Boosting Effect of Tuberculin Skin Testing: A Systematic Review
Background: Variability in interferon-gamma release assays (IGRAs) results for tuberculosis has implications for interpretation of results close to the cut-point, and for defining thresholds for test conversion and reversion. However, little is known about the within-subject variability (reproducibility) of IGRAs. Several national guidelines recommend a twostep testing procedure (tuberculin skin test [TST] followed by IGRA) for the diagnosis of LTBI. However, the effect of a preceding TST on subsequent IGRA results has been reported in studies with apparently conflicting results. Methodology/Findings: We conducted a systematic review to synthesize evidence on within-subject variability of IGRA results and the potential boosting effect of TST. We searched several databases and reviewed citations of previous reviews on IGRAs. We included studies using commercial IGRAs, in addition to non-commercial versions of the ELISPOT assay. Four studies, fulfilling our predefined criteria, examined within-subject variability and 13 studies evaluated TST effects on subsequent IGRA responses. Meta-analysis was not considered appropriate because of heterogeneity in study methods, assays, and populations. Although based on limited data, within-subject variability was present in all studies but the magnitude varied (16-80%) across studies. A TST induced ‘‘boosting’ ’ of IGRA responses was demonstrated in several studies and although more pronounced in IGRA-positive (i.e. sensitized) individuals, also occurred in a smaller but not insignificant proportion of IGRA-negative subjects. The TST appeared to affect IGRA responses only after 3 days and may apparentl
Phases of the one-dimensional Bose-Hubbard model
The zero-temperature phase diagram of the one-dimensional Bose-Hubbard model
with nearest-neighbor interaction is investigated using the Density-Matrix
Renormalization Group. Recently normal phases without long-range order have
been conjectured between the charge density wave phase and the superfluid phase
in one-dimensional bosonic systems without disorder. Our calculations
demonstrate that there is no intermediate phase in the one-dimensional
Bose-Hubbard model but a simultaneous vanishing of crystalline order and
appearance of superfluid order. The complete phase diagrams with and without
nearest-neighbor interaction are obtained. Both phase diagrams show reentrance
from the superfluid phase to the insulator phase.Comment: Revised version, 4 pages, 5 figure
Island diffusion on metal fcc(100) surfaces
We present Monte Carlo simulations for the size and temperature dependence of
the diffusion coefficient of adatom islands on the Cu(100) surface. We show
that the scaling exponent for the size dependence is not a constant but a
decreasing function of the island size and approaches unity for very large
islands. This is due to a crossover from periphery dominated mass transport to
a regime where vacancies diffuse inside the island. The effective scaling
exponents are in good agreement with theory and experiments.Comment: 13 pages, 2 figures, to be published in Phys. Rev. Let
Epitaxial growth of Cu on Cu(001): experiments and simulations
A quantitative comparison between experimental and Monte Carlo simulation
results for the epitaxial growth of Cu/Cu(001) in the submonolayer regime is
presented. The simulations take into account a complete set of hopping
processes whose activation energies are derived from semi-empirical
calculations using the embedded-atom method. The island separation is measured
as a function of the incoming flux and the temperature. A good quantitative
agreement between the experiment and simulation is found for the island
separation, the activation energies for the dominant processes, and the
exponents that characterize the growth. The simulation results are then
analyzed at lower coverages, which are not accessible experimentally, providing
good agreement with theoretical predictions as well.Comment: Latex document. 7 pages. 3 embedded figures in separate PS files. One
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Critical behavior of the 3-state Potts model on Sierpinski carpet
We study the critical behavior of the 3-state Potts model, where the spins
are located at the centers of the occupied squares of the deterministic
Sierpinski carpet. A finite-size scaling analysis is performed from Monte Carlo
simulations, for a Hausdorff dimension . The phase
transition is shown to be a second order one. The maxima of the susceptibility
of the order parameter follow a power law in a very reliable way, which enables
us to calculate the ratio of the exponents . We find that the
scaling corrections affect the behavior of most of the thermodynamical
quantities. However, the sequence of intersection points extracted from the
Binder's cumulant provides bounds for the critical temperature. We are able to
give the bounds for the exponent as well as for the ratio of the
exponents , which are compatible with the results calculated from
the hyperscaling relation.Comment: 13 pages, 4 figure
Is there a vortex-glass transition in high-temperature superconductors?
We show that DC voltage versus current measurements of a YBCO micro-bridge in
a magnetic field can be collapsed onto scaling functions proposed by Fisher,
Fisher, and Huse, as is widely reported in the literature. We find, however,
that good data collapse is achieved for a wide range of critical exponents and
temperatures. These results strongly suggest that agreement with scaling alone
does not prove the existence of a phase transition. We propose a criterion to
determine if the data collapse is valid, and thus if a phase transition occurs.
To our knowledge, none of the data reported in the literature meet our
criterion.Comment: 4 pages, 4 figure
Performance of LED-Based Fluorescence Microscopy to Diagnose Tuberculosis in a Peripheral Health Centre in Nairobi.
Sputum microscopy is the only tuberculosis (TB) diagnostic available at peripheral levels of care in resource limited countries. Its sensitivity is low, particularly in high HIV prevalence settings. Fluorescence microscopy (FM) can improve performance of microscopy and with the new light emitting diode (LED) technologies could be appropriate for peripheral settings. The study aimed to compare the performance of LED-FM versus Ziehl-Neelsen (ZN) microscopy and to assess feasibility of LED-FM at a low level of care in a high HIV prevalence country
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