7,498 research outputs found
Effects of counterion fluctuations in a polyelectrolyte brush
We investigate the effect of counterion fluctuations in a single
polyelectrolyte brush in the absence of added salt by systematically expanding
the counterion free energy about Poisson-Boltzmann mean field theory. We find
that for strongly charged brushes, there is a collapse regime in which the
brush height decreases with increasing charge on the polyelectrolyte chains.
The transition to this collapsed regime is similar to the liquid-gas
transition, which has a first-order line terminating at a critical point. We
find that for monovalent counterions the transition is discontinuous in theta
solvent, while for multivalent counterions the transition is generally
continuous. For collapsed brushes, the brush height is not independent of
grafting density as it is for osmotic brushes, but scales linear with it.Comment: 9 pages, 9 figure
The Lagrange Equilibrium Points L_4 and L_5 in a Black Hole Binary System
We calculate the location and stability of the L_4 and L_5 Lagrange
equilibrium points in the circular restricted three-body problem as the binary
system evolves via gravitational radiation losses. Relative to the purely
Newtonian case, we find that the L_4 equilibrium point moves towards the
secondary mass and becomes slightly less stable, while the L_5 point moves away
from the secondary and gains in stability. We discuss a number of astrophysical
applications of these results, in particular as a mechanism for producing
electromagnetic counterparts to gravitational-wave signals.Comment: 10 pages, 4 figures, submitted to ApJ; comments welcom
Gravitational waves from spinning eccentric binaries
This paper is to introduce a new software called CBwaves which provides a
fast and accurate computational tool to determine the gravitational waveforms
yielded by generic spinning binaries of neutron stars and/or black holes on
eccentric orbits. This is done within the post-Newtonian (PN) framework by
integrating the equations of motion and the spin precession equations while the
radiation field is determined by a simultaneous evaluation of the analytic
waveforms. In applying CBwaves various physically interesting scenarios have
been investigated. In particular, we have studied the appropriateness of the
adiabatic approximation, and justified that the energy balance relation is
indeed insensitive to the specific form of the applied radiation reaction term.
By studying eccentric binary systems it is demonstrated that circular template
banks are very ineffective in identifying binaries even if they possess tiny
residual orbital eccentricity. In addition, by investigating the validity of
the energy balance relation we show that, on contrary to the general
expectations, the post-Newtonian approximation should not be applied once the
post-Newtonian parameter gets beyond the critical value .
Finally, by studying the early phase of the gravitational waves emitted by
strongly eccentric binary systems---which could be formed e.g. in various
many-body interactions in the galactic halo---we have found that they possess
very specific characteristics which may be used to identify these type of
binary systems.Comment: 37 pages, 18 figures, submitted to Class. Quantum Gra
Older US Emergency Department Patients Are Less Likely to Receive Pain Medication Than Younger Patients: Results From a National Survey
The purpose of this study is to determine whether older adults presenting to the Emergency Department (ED) with pain are less likely to receive pain medication than younger adults
Attention on Weak Ties in Social and Communication Networks
Granovetter's weak tie theory of social networks is built around two central
hypotheses. The first states that strong social ties carry the large majority
of interaction events; the second maintains that weak social ties, although
less active, are often relevant for the exchange of especially important
information (e.g., about potential new jobs in Granovetter's work). While
several empirical studies have provided support for the first hypothesis, the
second has been the object of far less scrutiny. A possible reason is that it
involves notions relative to the nature and importance of the information that
are hard to quantify and measure, especially in large scale studies. Here, we
search for empirical validation of both Granovetter's hypotheses. We find clear
empirical support for the first. We also provide empirical evidence and a
quantitative interpretation for the second. We show that attention, measured as
the fraction of interactions devoted to a particular social connection, is high
on weak ties --- possibly reflecting the postulated informational purposes of
such ties --- but also on very strong ties. Data from online social media and
mobile communication reveal network-dependent mixtures of these two effects on
the basis of a platform's typical usage. Our results establish a clear
relationships between attention, importance, and strength of social links, and
could lead to improved algorithms to prioritize social media content
Anyonic interferometry and protected memories in atomic spin lattices
Strongly correlated quantum systems can exhibit exotic behavior called
topological order which is characterized by non-local correlations that depend
on the system topology. Such systems can exhibit remarkable phenomena such as
quasi-particles with anyonic statistics and have been proposed as candidates
for naturally fault-tolerant quantum computation. Despite these remarkable
properties, anyons have never been observed in nature directly. Here we
describe how to unambiguously detect and characterize such states in recently
proposed spin lattice realizations using ultra-cold atoms or molecules trapped
in an optical lattice. We propose an experimentally feasible technique to
access non-local degrees of freedom by performing global operations on trapped
spins mediated by an optical cavity mode. We show how to reliably read and
write topologically protected quantum memory using an atomic or photonic qubit.
Furthermore, our technique can be used to probe statistics and dynamics of
anyonic excitations.Comment: 14 pages, 6 figure
Signalling and the Evolution of Cooperative Foraging in Dynamic Environments
Understanding cooperation in animal social groups remains a significant challenge for evolutionary theory. Observed behaviours that benefit others but incur some cost appear incompatible with classical notions of natural selection; however, these behaviours may be explained by concepts such as inclusive fitness, reciprocity, intra-specific mutualism or manipulation. In this work, we examine a seemingly altruistic behaviour, the active recruitment of conspecifics to a food resource through signalling. Here collective, cooperative behaviour may provide highly nonlinear benefits to individuals, since group functionality has the potential to be far greater than the sum of the component parts, for example by enabling the effective tracking of a dynamic resource. We show that due to this effect, signalling to others is an evolutionarily stable strategy under certain environmental conditions, even when there is a cost associated to this behaviour. While exploitation is possible, in the limiting case of a sparse, ephemeral but locally abundant nutrient source, a given environmental profile will support a fixed number of signalling individuals. Through a quantitative analysis, this effective carrying capacity for cooperation is related to the characteristic length and time scales of the resource field
Aspects of holography for theories with hyperscaling violation
We analyze various aspects of the recently proposed holographic theories with
general dynamical critical exponent z and hyperscaling violation exponent
. We first find the basic constraints on from the gravity
side, and compute the stress-energy tensor expectation values and scalar
two-point functions. Massive correlators exhibit a nontrivial exponential
behavior at long distances, controlled by . At short distance, the
two-point functions become power-law, with a universal form for .
Next, the calculation of the holographic entanglement entropy reveals the
existence of novel phases which violate the area law. The entropy in these
phases has a behavior that interpolates between that of a Fermi surface and
that exhibited by systems with extensive entanglement entropy. Finally, we
describe microscopic embeddings of some metrics into full
string theory models -- these metrics characterize large regions of the
parameter space of Dp-brane metrics for . For instance, the theory of
N D2-branes in IIA supergravity has z=1 and over a wide range
of scales, at large .Comment: 35 pages; v2: new references added; v3: proper reference [14] added;
v4: minor clarification
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