2,821 research outputs found
Coherent Population Trapping of Electron Spins in a Semiconductor
In high-purity n-type GaAs under strong magnetic field, we are able to
isolate a lambda system composed of two Zeeman states of neutral-donor bound
electrons and the lowest Zeeman state of bound excitons. When the two-photon
detuning of this system is zero, we observe a pronounced dip in the
excited-state photoluminescence indicating the creation of the coherent
population-trapped state. Our data are consistent with a steady-state
three-level density-matrix model. The observation of coherent population
trapping in GaAs indicates that this and similar semiconductor systems could be
used for various EIT-type experiments.Comment: 5 pages, 4 figures replaced 6/25/2007 with PRL versio
Genetic Correlations in Mutation Processes
We study the role of phylogenetic trees on correlations in mutation
processes. Generally, correlations decay exponentially with the generation
number. We find that two distinct regimes of behavior exist. For mutation rates
smaller than a critical rate, the underlying tree morphology is almost
irrelevant, while mutation rates higher than this critical rate lead to strong
tree-dependent correlations. We show analytically that identical critical
behavior underlies all multiple point correlations. This behavior generally
characterizes branching processes undergoing mutation.Comment: revtex, 8 pages, 2 fig
Enforced Bcl-2 Expression Inhibits Antigen-mediated Clonal Elimination of Peripheral B Cells in an Antigen Dose–dependent Manner and Promotes Receptor Editing in Autoreactive, Immature B Cells
The mechanisms that establish immune tolerance in immature and mature B cells appear to be distinct. Membrane-bound autoantigen is thought to induce developmental arrest and receptor editing in immature B cells, whereas mature B cells have shortened lifespans when exposed to the same stimulus. In this study, we used Eμ–bcl-2-22 transgenic (Tg) mice to test the prediction that enforced expression of the Bcl-2 apoptotic inhibitor in B cells would rescue mature, but not immature, B cells from tolerance induction. To monitor tolerance to the natural membrane autoantigen H-2Kb, we bred 3–83μδ (anti-Kk,b) Ig Tg mice to H-2b mice or to mice expressing transgene-driven Kb in the periphery. In 3–83μδ/bcl-2 Tg mice, deletion of autoreactive B cells induced by peripheral Kb antigen expression in the liver (MT-Kb Tg) or epithelia (KerIV-Kb Tg), was partly or completely inhibited, respectively. Furthermore, Bcl-2 protected peritoneal B-2 B cells from deletion mediated by acute antigen exposure, but this protection could be overcome by higher antigen dose. In contrast to its ability to block peripheral self-tolerance, Bcl-2 overexpression failed to inhibit central tolerance induced by bone marrow antigen expression, but instead, enhanced the receptor editing process. These studies indicate that apoptosis plays distinct roles in central and peripheral B cell tolerance
Temporally disordered Ising models
We present a study of the influence of different types of disorder on systems
in the Ising universality class by employing both a dynamical field theory
approach and extensive Monte Carlo simulations. We reproduce some well known
results for the case of quenched disorder (random temperature and random
field), and analyze the effect of four different types of time-dependent
disorder scarcely studied so far in the literature. Some of them are of obvious
experimental and theoretical relevance (as for example, globally fluctuating
temperatures or random fields). All the predictions coming from our field
theoretical analysis are fully confirmed by extensive simulations in two and
three dimensions, and novel qualitatively different, non-Ising transitions are
reported. Possible experimental setups designed to explore the described
phenomenologies are also briefly discussed.Comment: Submitted to Phys. Rev. E. Rapid Comm. 4 page
Exploring the implications of vision, appropriateness, and execution of organizational change
Avalanches in the lung: A statistical mechanical model
We study a statistical mechanical model for the dynamics of lung inflation
which incorporates recent experimental observations on the opening of
individual airways by a cascade or avalanche mechanism. Using an exact mapping
of the avalanche problem onto percolation on a Cayley tree, we analytically
derive the exponents describing the size distribution of the first avalanches
and test the analytical solution by numerical simulations. We find that the
tree-like structure of the airways together with the simplest assumptions
concerning opening threshold pressures of each airway, is sufficient to explain
the existence of power-law distributions observed experimentally.Comment: 4 pages, Figures avaliable by mail from [email protected], REVTE
Isospin splittings of doubly heavy baryons
The SELEX Collaboration has reported a very large isospin splitting of doubly
charmed baryons. We show that this effect would imply that the doubly charmed
baryons are very compact. One intriguing possibility is that such baryons have
a linear geometry Q-q-Q where the light quark q oscillates between the two
heavy quarks Q, analogous to a linear molecule such as carbon dioxide. However,
using conventional arguments, the size of a heavy-light hadron is expected to
be around 0.5 fm, much larger than the size needed to explain the observed
large isospin splitting. Assuming the distance between two heavy quarks is much
smaller than that between the light quark and a heavy one, the doubly heavy
baryons are related to the heavy mesons via heavy quark-diquark symmetry. Based
on this symmetry, we predict the isospin splittings for doubly heavy baryons
including Xi_{cc}, Xi_{bb} and Xi_{bc}. The prediction for the Xi_{cc} is much
smaller than the SELEX value. On the other hand, the Xi_{bb} baryons are
predicted to have an isospin splitting as large as (6.3\pm1.7) MeV. An
experimental study of doubly bottomed baryons is therefore very important to
better understand the structure of baryons with heavy quarks.Comment: 11 page
Organizational culture and effectiveness: A study of values, attitudes, and organizational outcomes
a b s t r a c t a r t i c l e i n f o Organizational culture is defined as a set of beliefs, values, and assumptions that are shared by members of an organization (Schein, 1985). These underlying values have an influence on the behavior of organizational members, as people rely on these values to guide their decisions and behaviors (Schein, 1985). Extrapolating from the influence culture has on the behavior of organizational members, much has been written about the impact of culture on an organization's effectiveness (e.g., Schein, 1985, Quinn, 1988. While the extant research is promising, more empirical evidence of the manner in which organizational culture impacts effectiveness is warranted. Previous research has explored the direct relationships between specific culture domains and specific effectiveness measures (e.g., Quinn and Spreitzer, 1991; Cameron and Freeman, 1991; Denison and Mishra, 1995; Denison, 1990). The purpose of this research is to delve deeper into the relationship between organizational culture and organizational effectiveness by exploring both direct and indirect effects. Siehl and Martin (1990) suggested that culture influences employee attitudes and that those attitudes, in turn, impact organizational effectiveness. We offer an empirical examination of this assertion by testing the mediating effect of employee satisfaction on the culture-effectiveness relationship. Frazier et al. (2004) describe mediating variables as constructs that "establish 'how' or 'why' one variable predicts or causes and outcome variable" (p. 116). Although the relationship between culture and effectiveness is relatively well established in the literature, "how" and "why" this relationship exists has not been adequately addressed. This manuscript attempts to begin to fill that void by exploring employee attitudes as one possible explanatory mechanism through which an organization's culture comes to impact its performance
The Ess/Type VII secretion system of Staphylococcus aureus shows unexpected genetic diversity
We thank the core sequencing and informatics teams at the Sanger Institute for their assistance and The Wellcome Trust for its support of the Sanger Institute Pathogen Genomics and Biology groups. SRH, JP and MTGH were supported by Wellcome Trust grant 098051. Bioinformatics and Computational Biology analyses were supported by the University of St Andrews Bioinformatics Unit that is funded by a Wellcome Trust ISSF award (grant 105621/Z/14/Z). SP is funded by the UKCRC Translational Infection Research Initiative, and the NIHR Cambridge Biomedical Research Centre. CPH is supported by the Wellcome Trust (grant number 104241/z/14/z) TP is a Royal Society/Wolfson Merit Award Holder.BACKGROUND: Type VII protein secretion (T7SS) is a specialised system for excreting extracellular proteins across bacterial cell membranes and has been associated with virulence in Staphylococcus aureus. The genetic diversity of the ess locus, which encodes the T7SS, and the functions of proteins encoded within it are poorly understood. RESULTS: We used whole genome sequence data from 153 isolates representative of the diversity of the species to investigate the genetic variability of T7SS across S. aureus. The ess loci were found to comprise of four distinct modules based on gene content and relative conservation. Modules 1 and 4, comprising of the 5' and 3' modules of the ess locus, contained the most conserved clusters of genes across the species. Module 1 contained genes encoding the secreted protein EsxA, and the EsaAB and EssAB components of the T7SS machinery, and Module 4 contained two functionally uncharacterized conserved membrane proteins. Across the species four variants of Module 2 were identified containing the essC gene, each of which was associated with a specific group of downstream genes. The most diverse module of the ess locus was Module 3 comprising a highly variable arrangement of hypothetical proteins. RNA-Seq was performed on representatives of the four Module 2 variants and demonstrated strain-specific differences in the levels of transcription in the conserved Module 1 components and transcriptional linkage Module 2, and provided evidence of the expression of genes the variable regions of the ess loci. CONCLUSIONS: The ess locus of S. aureus exhibits modularity and organisational variation across the species and transcriptional variation. In silico analysis of ess loci encoded hypothetical proteins identified potential novel secreted substrates for the T7SS. The considerable variety in operon arrangement between otherwise closely related isolates provides strong evidence for recombination at this locus. Comparison of these recombination regions with each other, and with the genomes of other Staphylococcal species, failed to identify evidence of intra- and inter-species recombination, however the analysis identified a novel T7SS in another pathogenic staphylococci, Staphylococcus lugdunensis.Publisher PDFPeer reviewe
Optimal Path and Minimal Spanning Trees in Random Weighted Networks
We review results on the scaling of the optimal path length in random
networks with weighted links or nodes. In strong disorder we find that the
length of the optimal path increases dramatically compared to the known small
world result for the minimum distance. For Erd\H{o}s-R\'enyi (ER) and scale
free networks (SF), with parameter (), we find that the
small-world nature is destroyed. We also find numerically that for weak
disorder the length of the optimal path scales logaritmically with the size of
the networks studied. We also review the transition between the strong and weak
disorder regimes in the scaling properties of the length of the optimal path
for ER and SF networks and for a general distribution of weights, and suggest
that for any distribution of weigths, the distribution of optimal path lengths
has a universal form which is controlled by the scaling parameter
where plays the role of the disorder strength, and
is the length of the optimal path in strong disorder. The
relation for is derived analytically and supported by numerical
simulations. We then study the minimum spanning tree (MST) and show that it is
composed of percolation clusters, which we regard as "super-nodes", connected
by a scale-free tree. We furthermore show that the MST can be partitioned into
two distinct components. One component the {\it superhighways}, for which the
nodes with high centrality dominate, corresponds to the largest cluster at the
percolation threshold which is a subset of the MST. In the other component,
{\it roads}, low centrality nodes dominate. We demonstrate the significance
identifying the superhighways by showing that one can improve significantly the
global transport by improving a very small fraction of the network.Comment: review, accepted at IJB
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