13,094 research outputs found
Nonequilibrium Stationary States and Phase Transitions in Directed Ising Models
We study the nonequilibrium properties of directed Ising models with non
conserved dynamics, in which each spin is influenced by only a subset of its
nearest neighbours. We treat the following models: (i) the one-dimensional
chain; (ii) the two-dimensional square lattice; (iii) the two-dimensional
triangular lattice; (iv) the three-dimensional cubic lattice. We raise and
answer the question: (a) Under what conditions is the stationary state
described by the equilibrium Boltzmann-Gibbs distribution? We show that for
models (i), (ii), and (iii), in which each spin "sees" only half of its
neighbours, there is a unique set of transition rates, namely with exponential
dependence in the local field, for which this is the case. For model (iv), we
find that any rates satisfying the constraints required for the stationary
measure to be Gibbsian should satisfy detailed balance, ruling out the
possibility of directed dynamics. We finally show that directed models on
lattices of coordination number with exponential rates cannot
accommodate a Gibbsian stationary state. We conjecture that this property
extends to any form of the rates. We are thus led to the conclusion that
directed models with Gibbsian stationary states only exist in dimension one and
two. We then raise the question: (b) Do directed Ising models, augmented by
Glauber dynamics, exhibit a phase transition to a ferromagnetic state? For the
models considered above, the answers are open problems, to the exception of the
simple cases (i) and (ii). For Cayley trees, where each spin sees only the
spins further from the root, we show that there is a phase transition provided
the branching ratio, , satisfies
The Micro Slit Gas Detector
We describe the first tests with a new proportional gas detector. Its
geometry consists in slits opened in a copper metallized kapton foil with 30
micron anode strips suspended in these openings. In this way the multiplication
process is similar to a standard MSGC. The fundamental difference is the
absence of an insulating substrate around the anode. Also the material budget
is significantly reduced, and the problems related to charging-up or
polarization are removed. Ageing properties of this detector are under study.Comment: 13 pages tex file, 10 figures ep
Tracking quasi-classical chaos in ultracold boson gases
We study the dynamics of a ultra-cold boson gas in a lattice submitted to a
constant force. We track the route of the system towards chaos created by the
many-body-induced nonlinearity and show that relevant information can be
extracted from an experimentally accessible quantity, the gas mean position.
The threshold nonlinearity for the appearance of chaotic behavior is deduced
from KAM arguments and agrees with the value obtained by calculating the
associated Lyapunov exponent.Comment: 4 pages, revtex4, submitted to PR
On the backreaction of frame dragging
The backreaction on black holes due to dragging heavy, rather than test,
objects is discussed. As a case study, a regular black Saturn system where the
central black hole has vanishing intrinsic angular momentum, J^{BH}=0, is
considered. It is shown that there is a correlation between the sign of two
response functions. One is interpreted as a moment of inertia of the black ring
in the black Saturn system. The other measures the variation of the black ring
horizon angular velocity with the central black hole mass, for fixed ring mass
and angular momentum. The two different phases defined by these response
functions collapse, for small central black hole mass, to the thin and fat ring
phases. In the fat phase, the zero area limit of the black Saturn ring has
reduced spin j^2>1, which is related to the behaviour of the ring angular
velocity. Using the `gravitomagnetic clock effect', for which a universality
property is exhibited, it is shown that frame dragging measured by an
asymptotic observer decreases, in both phases, when the central black hole mass
increases, for fixed ring mass and angular momentum. A close parallelism
between the results for the fat phase and those obtained recently for the
double Kerr solution is drawn, considering also a regular black Saturn system
with J^{BH}\neq 0.Comment: 18 pages, 8 figure
Characterizing Quantum Properties of a Measurement Apparatus: Insights from the Retrodictive Approach
Using the retrodictive approach of quantum physics, we show that the state
retrodicted from the response of a measurement apparatus is a convenient tool
to fully characterize its quantum properties. We translate in terms of this
state some interesting aspects of the quantum behavior of a detector, such as
the non-classicality or the non-gaussian character of its measurements. We also
introduce estimators - the projectivity, the ideality, the fidelity or the
detectivity of measurements perfomed by the apparatus - which directly follow
from the retrodictive approach. Beyond their fundamental significance for
describing general quantum measurements, these properties are crucial in
several protocols, in particular in the conditional preparation of
non-classical states of light or in measurement-driven quantum information
processing
Observer with a constant proper acceleration
Relying on the equivalence principle, a first approach of the general theory
of relativity is presented using the spacetime metric of an observer with a
constant proper acceleration. Within this non inertial frame, the equation of
motion of a freely moving object is studied and the equation of motion of a
second accelerated observer with the same proper acceleration is examined. A
comparison of the metric of the accelerated observer with the metric due to a
gravitational field is also performed.Comment: 5 figure
Evaluating the effectiveness of the Emergency Neurological Life Support educational framework in low-income countries.
BackgroundThe Emergency Neurological Life Support (ENLS) is an educational initiative designed to improve the acute management of neurological injuries. However, the applicability of the course in low-income countries in unknown. We evaluated the impact of the course on knowledge, decision-making skills and preparedness to manage neurological emergencies in a resource-limited country.MethodsA prospective cohort study design was implemented for the first ENLS course held in Asia. Knowledge and decision-making skills for neurological emergencies were assessed at baseline, post-course and at 6 months following course completion. To determine perceived knowledge and preparedness, data were collected using surveys administered immediately post-course and 6 months later.ResultsA total of 34 acute care physicians from across Nepal attended the course. Knowledge and decision-making skills significantly improved following the course (p=0.0008). Knowledge and decision-making skills remained significantly improved after 6 months, compared with before the course (p=0.02), with no significant loss of skills immediately following the course to the 6-month follow-up (p=0.16). At 6 months, the willingness to participate in continuing medical education activities remained evident, with 77% (10/13) of participants reporting a change in their clinical practice and decision-making, with the repeated use of ENLS protocols as the main driver of change.ConclusionsUsing the ENLS framework, neurocritical care education can be delivered in low-income countries to improve knowledge uptake, with evidence of knowledge retention up to 6 months
High-Resolution X-ray Spectroscopy of SNR 1987A: Chandra LETG and HETG Observations in 2007
We present an extended analysis of the deep Chandra LETG and HETG
observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007.
The global fits to the grating spectra show that the temperature of the X-ray
emitting plasma in the slower shocks in this system has remained stable for the
last three years, while that in the faster shocks has decreased. This
temperature evolution is confirmed by the first light curves of strong X-ray
emission lines and their ratios. On the other hand, bulk gas velocities
inferred from the X-ray line profiles are too low to account for the post-shock
plasma temperatures inferred from spectral fits. This suggests that the X-ray
emission comes from gas that has been shocked twice, first by the blast wave
and again by shocks reflected from the inner ring of SNR 1987A. A new model
that takes these considerations into account gives support to this physical
picture.Comment: 36 pages, 10 figures, Accepted for publication in Ap
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