30 research outputs found
Dealloying of Platinum-Aluminum Thin Films Part I. Dynamics of Pattern Formation
Applying focused ion beam (FIB) nanotomography and Rutherford backscattering
spectroscopy (RBS) to dealloyed platinum-aluminum thin films an in-depth
analysis of the dominating physical mechanisms of porosity formation during the
dealloying process is performed. The dynamical porosity formation due to the
dissolution of the less noble aluminum in the alloy is treated as result of a
reaction-diffusion system. The RBS analysis yields that the porosity formation
is mainly caused by a linearly propagating diffusion front, i.e. the
liquid/solid interface, with a uniform speed of 42(3) nm/s when using a 4M
aqueous NaOH solution at room temperature. The experimentally observed front
evolution is captured by the normal diffusive
Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP) equation and can be interpreted
as a branching random walk phenomenon. The etching front produces a gradual
porosity with an enhanced porosity in the surface-near regions of the thin film
due to prolonged exposure of the alloy to the alkaline solution.Comment: 4 pages, 5 figure
Physical realization of a quantum spin liquid based on a novel frustration mechanism
Unlike conventional magnets where the magnetic moments are partially or
completely static in the ground state, in a quantum spin liquid they remain in
collective motion down to the lowest temperatures. The importance of this state
is that it is coherent and highly entangled without breaking local symmetries.
Such phenomena is usually sought in simple lattices where antiferromagnetic
interactions and/or anisotropies that favor specific alignments of the magnetic
moments are "frustrated" by lattice geometries incompatible with such order
e.g. triangular structures. Despite an extensive search among such compounds,
experimental realizations remain very few. Here we describe the investigation
of a novel, unexplored magnetic system consisting of strong ferromagnetic and
weaker antiferromagnetic isotropic interactions as realized by the compound
CaCrO. Despite its exotic structure we show both
experimentally and theoretically that it displays all the features expected of
a quantum spin liquid including coherent spin dynamics in the ground state and
the complete absence of static magnetism.Comment: Modified version accepted in Nature Physic
Search for Gravitational Waves from Low Mass Compact Binary Coalescence in LIGO's Sixth Science Run and Virgo's Science Runs 2 and 3
We report on a search for gravitational waves from coalescing compact
binaries using LIGO and Virgo observations between July 7, 2009 and October 20,
2010. We searched for signals from binaries with total mass between 2 and 25
solar masses; this includes binary neutron stars, binary black holes, and
binaries consisting of a black hole and neutron star. The detectors were
sensitive to systems up to 40 Mpc distant for binary neutron stars, and further
for higher mass systems. No gravitational-wave signals were detected. We report
upper limits on the rate of compact binary coalescence as a function of total
mass, including the results from previous LIGO and Virgo observations. The
cumulative 90%-confidence rate upper limits of the binary coalescence of binary
neutron star, neutron star- black hole and binary black hole systems are 1.3 x
10^{-4}, 3.1 x 10^{-5} and 6.4 x 10^{-6} Mpc^{-3}yr^{-1}, respectively. These
upper limits are up to a factor 1.4 lower than previously derived limits. We
also report on results from a blind injection challenge.Comment: 11 pages, 5 figures. For a repository of data used in the
publication, go to:
. Also see the
announcement for this paper on ligo.org at:
<http://www.ligo.org/science/Publication-S6CBCLowMass/index.php
Implementation and testing of the first prompt search for gravitational wave transients with electromagnetic counterparts
Aims. A transient astrophysical event observed in both gravitational wave
(GW) and electromagnetic (EM) channels would yield rich scientific rewards. A
first program initiating EM follow-ups to possible transient GW events has been
developed and exercised by the LIGO and Virgo community in association with
several partners. In this paper, we describe and evaluate the methods used to
promptly identify and localize GW event candidates and to request images of
targeted sky locations.
Methods. During two observing periods (Dec 17 2009 to Jan 8 2010 and Sep 2 to
Oct 20 2010), a low-latency analysis pipeline was used to identify GW event
candidates and to reconstruct maps of possible sky locations. A catalog of
nearby galaxies and Milky Way globular clusters was used to select the most
promising sky positions to be imaged, and this directional information was
delivered to EM observatories with time lags of about thirty minutes. A Monte
Carlo simulation has been used to evaluate the low-latency GW pipeline's
ability to reconstruct source positions correctly.
Results. For signals near the detection threshold, our low-latency algorithms
often localized simulated GW burst signals to tens of square degrees, while
neutron star/neutron star inspirals and neutron star/black hole inspirals were
localized to a few hundred square degrees. Localization precision improves for
moderately stronger signals. The correct sky location of signals well above
threshold and originating from nearby galaxies may be observed with ~50% or
better probability with a few pointings of wide-field telescopes.Comment: 17 pages. This version (v2) includes two tables and 1 section not
included in v1. Accepted for publication in Astronomy & Astrophysic
Micro-solid oxide fuel cells as power supply for small portable electronic equipment
Micro-solid oxide fuel cell (SOFC) systems are anticipated for powering small, portable electronic devices, such as laptop, personal digital assistant (PDA), medical and industrial accessories. It is predicted that micro-SOFC systems have a 2-4 higher energy density than Li-ion batteries [1]. However, literature mainly focuses on the fabrication and characterization of thin films and membranes for micro-SOFC systems [2-12]; the entire system approach is not yet studied in detail. We will therefore discuss in this paper the entire approach from the fabrication of thin films and membranes up to the complete system, including fuel processing, thermal management and integration
Neisseria meningitidis Differentially Controls Host Cell Motility through PilC1 and PilC2 Components of Type IV Pili
Neisseria meningitidis is a strictly human pathogen that has two facets since asymptomatic carriage can unpredictably turn into fulminant forms of infection. Meningococcal pathogenesis relies on the ability of the bacteria to break host epithelial or endothelial cellular barriers. Highly restrictive, yet poorly understood, mechanisms allow meningococcal adhesion to cells of only human origin. Adhesion of encapsulated and virulent meningococci to human cells relies on the expression of bacterial type four pili (T4P) that trigger intense host cell signalling. Among the components of the meningococcal T4P, the concomitantly expressed PilC1 and PilC2 proteins regulate pili exposure at the bacterial surface, and until now, PilC1 was believed to be specifically responsible for T4P-mediated meningococcal adhesion to human cells. Contrary to previous reports, we show that, like PilC1, the meningococcal PilC2 component is capable of mediating adhesion to human ME180 epithelial cells, with cortical plaque formation and F-actin condensation. However, PilC1 and PilC2 promote different effects on infected cells. Cellular tracking analysis revealed that PilC1-expressing meningococci caused a severe reduction in the motility of infected cells, which was not the case when cells were infected with PilC2-expressing strains. The amount of both total and phosphorylated forms of EGFR was dramatically reduced in cells upon PilC1-mediated infection. In contrast, PilC2-mediated infection did not notably affect the EGFR pathway, and these specificities were shared among unrelated meningococcal strains. These results suggest that meningococci have evolved a highly discriminative tool for differential adhesion in specific microenvironments where different cell types are present. Moreover, the fine-tuning of cellular control through the combined action of two concomitantly expressed, but distinctly regulated, T4P-associated variants of the same molecule (i.e. PilC1 and PilC2) brings a new model to light for the analysis of the interplay between pathogenic bacteria and human host cells
Sensitivity Achieved by the LIGO and Virgo Gravitational Wave Detectors during LIGO's Sixth and Virgo's Second and Third Science Runs
We summarize the sensitivity achieved by the LIGO and Virgo gravitational wave detectors for low-mass compact binary coalescence (CBC) searches during LIGO's sixth science run and Virgo's second and third science runs. We present strain noise power spectral densities (PSDs) which are representative of the typical performance achieved by the detectors in these science runs. The data presented here and in the accompanying web-accessible data files are intended to be released to the public as a summary of detector performance for low-mass CBC searches during S6 and VSR2-3
All-sky Search for Periodic Gravitational Waves in the Full S5 LIGO Data
We report on an all-sky search for periodic gravitational waves in the
frequency band 50-800 Hz and with the frequency time derivative in the range of
0 through -6e-9 Hz/s. Such a signal could be produced by a nearby spinning and
slightly non-axisymmetric isolated neutron star in our galaxy. After recent
improvements in the search program that yielded a 10x increase in computational
efficiency, we have searched in two years of data collected during LIGO's fifth
science run and have obtained the most sensitive all-sky upper limits on
gravitational wave strain to date. Near 150 Hz our upper limit on worst-case
linearly polarized strain amplitude is 1e-24, while at the high end of
our frequency range we achieve a worst-case upper limit of 3.8e-24 for all
polarizations and sky locations. These results constitute a factor of two
improvement upon previously published data. A new detection pipeline utilizing
a Loosely Coherent algorithm was able to follow up weaker outliers, increasing
the volume of space where signals can be detected by a factor of 10, but has
not revealed any gravitational wave signals. The pipeline has been tested for
robustness with respect to deviations from the model of an isolated neutron
star, such as caused by a low-mass or long-period binary companion.Comment: 18 page
The spotted gar genome illuminates vertebrate evolution and facilitates human-teleost comparisons
To connect human biology to fish biomedical models, we sequenced the genome of spotted gar (Lepisosteus oculatus), whose lineage diverged from teleosts before teleost genome duplication (TGD). The slowly evolving gar genome has conserved in content and size many entire chromosomes from bony vertebrate ancestors. Gar bridges teleosts to tetrapods by illuminating the evolution of immunity, mineralization and development (mediated, for example, by Hox, ParaHox and microRNA genes). Numerous conserved noncoding elements (CNEs; often cis regulatory) undetectable in direct human-teleost comparisons become apparent using gar: functional studies uncovered conserved roles for such cryptic CNEs, facilitating annotation of sequences identified in human genome-wide association studies. Transcriptomic analyses showed that the sums of expression domains and expression levels for duplicated teleost genes often approximate the patterns and levels of expression for gar genes, consistent with subfunctionalization. The gar genome provides a resource for understanding evolution after genome duplication, the origin of vertebrate genomes and the function of human regulatory sequences