6,144 research outputs found
Interface states of quantum spin systems
We review recent results as well as ongoing work and open problems concerning
interface states in quantum spin systems at zero and finite temperature.Comment: 10 pages, LaTe
A fast high-order solver for problems of scattering by heterogeneous bodies
A new high-order integral algorithm for the solution of scattering problems by heterogeneous bodies is presented. Here, a scatterer is described by a (continuously or discontinuously) varying refractive index n(x) within a two-dimensional (2D) bounded region; solutions of the associated Helmholtz equation under given incident fields are then obtained by high-order inversion of the Lippmann-Schwinger integral equation. The algorithm runs in O(Nlog(N)) operations where N is the number of discretization points. A wide variety of numerical examples provided include applications to highly singular geometries, high-contrast configurations, as well as acoustically/electrically large problems for which supercomputing resources have been used recently. Our method provides highly accurate solutions for such problems on small desktop computers in CPU times of the order of seconds
Rigidity of interfaces in the Falicov-Kimball model
We analyze the thermodynamic properties of interfaces in the
three-dimensional Falicov Kimball model, which can be viewed as a primitive
quantum lattice model of crystalline matter. In the strong coupling limit, the
ionic subsystem of this model is governed by the Hamiltonian of an effective
classical spin model whose leading part is the Ising Hamiltonian. We prove that
the 100 interface in this model, at half-filling, is rigid, as in the
three-dimensional Ising model. However, despite the above similarities with the
Ising model, the thermodynamic properties of its 111 interface are very
different. We prove that even though this interface is expected to be unstable
for the Ising model, it is stable for the Falicov Kimball model at sufficiently
low temperatures. This rigidity results from a phenomenon of "ground state
selection" and is a consequence of the Fermi statistics of the electrons in the
model.Comment: 79 pages, 9 figures included as ps-files, appendix added in revisio
Efficient genetic algorithms for solving hard constrained optimization problems
This paper studies many Genetic Algorithm strategies
to solve hard-constrained optimization problems. It investigates the role of various genetic operators to avoid premature convergence. In particular, an analysis of niching methods is carried out on a simple function to show advantages and drawbacks of each of them. Comparisons are also performed on an original benchmark based on an electrode shape optimization technique coupled with a charge simulation metho
Niching genetic algorithms for optimization in electromagnetics. I. Fundamentals
Niching methods extend genetic algorithms and permit the investigation of multiple optimal solutions in the search space. In this paper, we review and discuss various strategies of niching for optimization in electromagnetics. Traditional mathematical problems and an electromagnetic benchmark are solved using niching genetic algorithms to show their interest in real world optimization
Catalog Dynamics: Impact of Content Publishing and Perishing on the Performance of a LRU Cache
The Internet heavily relies on Content Distribution Networks and transparent
caches to cope with the ever-increasing traffic demand of users. Content,
however, is essentially versatile: once published at a given time, its
popularity vanishes over time. All requests for a given document are then
concentrated between the publishing time and an effective perishing time.
In this paper, we propose a new model for the arrival of content requests,
which takes into account the dynamical nature of the content catalog. Based on
two large traffic traces collected on the Orange network, we use the
semi-experimental method and determine invariants of the content request
process. This allows us to define a simple mathematical model for content
requests; by extending the so-called "Che approximation", we then compute the
performance of a LRU cache fed with such a request process, expressed by its
hit ratio. We numerically validate the good accuracy of our model by comparison
to trace-based simulation.Comment: 13 Pages, 9 figures. Full version of the article submitted to the ITC
2014 conference. Small corrections in the appendix from the previous versio
Stabilising a nulling interferometer using optical path difference dithering
Context. Nulling interferometry has been suggested as the underlying
principle for the Darwin and TPF-I exoplanet research missions. Aims. There are
constraints both on the mean value of the nulling ratio, and on its stability.
Instrument instability noise is most detrimental to the stability of the
nulling performance. Methods. We applied a modified version of the classical
dithering technique to the optical path difference in the scientific beam.
Results. Using only this method, we repeatedly stabilised the dark fringe for
several hours. This method alone sufficed to remove the 1/ f component of the
noise in our setup for periods of 10 minutes, typically. These results indicate
that performance stability may be maintained throughout the long-duration data
acquisitions typical of exoplanet spectroscopy. Conclusions. We suggest that
further study of possible stabilisation strategies should be an integral part
of Darwin/TPF-I research and developmen
Discrete ply model of circular pull-through test of fasteners in laminates
In aeronautical structures, assemblies with thin laminates are becoming increasingly usual, especially for fuselage design. In these structures, out-of-plane loads can appear in bolted joints and can lead to progressive punching of the fastenerâs head in the laminate resulting, in some cases, in a failure mode called pull-through [1]. This complex phenomenon, which occurs in assemblies, was studied firstly by using a simplified ââcircularââ pull-through test method. Qualitative micrographic examinations showed damage very similar to that observed in impacted specimens. The research presented here extends the Discrete Ply Model Method (DPM) developed by Bouvet et al. [2] to this case. The finite elements model is based on a particular mesh taking ply orientations into account. Cohesive elements are placed at the interfaces between solid elements to represent matrix cracks and delamination, thus allowing the natural coupling between these two damage modes to be represented. The model shows good correlation with test results, in terms of load/displacement curve, and correct prediction of the damage map until failure, including the
splitting phenomenon
Get paid more, work more? Lessons from French physicians' labour supply responses to hypothetic fee increases
This paper is devoted to the analysis of the General Practitioners' (GPs) labour supply, specifically focusing on the physicians' labour supply responses to higher compensations. This analysis is mainly aimed at challenging the reality of a âbackward bending' form for the labour supply of GPs. Because GPs' fees only evolve very slowly and are mainly fixed by the National Health Insurance Fund, we designed a contingent valuation survey in which hypothetical fee increases are randomly submitted to GPs. Empirical evidence from 1,400 French GPs supports the hypothesis of a negative slope for the GPs' labour supply curve. Therefore, increasing the supply of physicians' services through an increase in fees is not a feasible policy.General practitioners; contingent valuation; price of leisure; labour supply; backward bending curve
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