27,862 research outputs found
Counting (3+1) - Avoiding permutations
A poset is {\it (\3+\1)-free} if it contains no induced subposet isomorphic
to the disjoint union of a 3-element chain and a 1-element chain. These posets
are of interest because of their connection with interval orders and their
appearance in the (\3+\1)-free Conjecture of Stanley and Stembridge. The
dimension 2 posets are exactly the ones which have an associated
permutation where in if and only if as integers and
comes before in the one-line notation of . So we say that a
permutation is {\it (\3+\1)-free} or {\it (\3+\1)-avoiding} if its
poset is (\3+\1)-free. This is equivalent to avoiding the permutations
2341 and 4123 in the language of pattern avoidance. We give a complete
structural characterization of such permutations. This permits us to find their
generating function.Comment: 17 page
Aging, rejuvenation and memory effects in re-entrant ferromagnets
We have studied the slow dynamics of the ferromagnetic phases of the
re-entrant CdCr_{2x}In_{2-2x}S_4 system for 0.85<x<=1 by means of low frequency
ac susceptibility and magnetization measurements. Experimental procedures
widely used in the investigation of the out-of-equilibrium dynamics of spin
glasses (such as the x=0.85 compound) have been applied to search for aging,
rejuvenation and memory effects, and to test their dependence on the disorder
introduced by dilution of the magnetic ions. Whereas the rejuvenation effect is
found in all studied samples, the memory effect is clearly enhanced for
increasing dilutions. The results support a description of aging in both
ferromagnetic and re-entrant spin-glass phases in terms of hierarchical
reconformations of domain walls pinned by the disorder.Comment: Service de Physique de l'Etat Condense, DRECAM, DSM, CEA Saclay,91191
Gif sur Yvette Cedex, France, 9 pages, including 7 figures, To appear in Eur.
Phys. J. B (2002
PyFR: An Open Source Framework for Solving Advection-Diffusion Type Problems on Streaming Architectures using the Flux Reconstruction Approach
High-order numerical methods for unstructured grids combine the superior
accuracy of high-order spectral or finite difference methods with the geometric
flexibility of low-order finite volume or finite element schemes. The Flux
Reconstruction (FR) approach unifies various high-order schemes for
unstructured grids within a single framework. Additionally, the FR approach
exhibits a significant degree of element locality, and is thus able to run
efficiently on modern streaming architectures, such as Graphical Processing
Units (GPUs). The aforementioned properties of FR mean it offers a promising
route to performing affordable, and hence industrially relevant,
scale-resolving simulations of hitherto intractable unsteady flows within the
vicinity of real-world engineering geometries. In this paper we present PyFR,
an open-source Python based framework for solving advection-diffusion type
problems on streaming architectures using the FR approach. The framework is
designed to solve a range of governing systems on mixed unstructured grids
containing various element types. It is also designed to target a range of
hardware platforms via use of an in-built domain specific language based on the
Mako templating engine. The current release of PyFR is able to solve the
compressible Euler and Navier-Stokes equations on grids of quadrilateral and
triangular elements in two dimensions, and hexahedral elements in three
dimensions, targeting clusters of CPUs, and NVIDIA GPUs. Results are presented
for various benchmark flow problems, single-node performance is discussed, and
scalability of the code is demonstrated on up to 104 NVIDIA M2090 GPUs. The
software is freely available under a 3-Clause New Style BSD license (see
www.pyfr.org)
Ageing and relaxation times in disordered insulators
We focus on the slow relaxations observed in the conductance of disordered
insulators at low temperature (especially granular aluminum films). They
manifest themselves as a temporal logarithmic decrease of the conductance after
a quench from high temperatures and the concomitant appearance of a field
effect anomaly centered on the gate voltage maintained. We are first interested
in ageing effects, i.e. the age dependence of the dynamical properties of the
system. We stress that the formation of a second field effect anomaly at a
different gate voltage is not a "history free" logarithmic (lnt) process, but
departs from lnt in a way which encodes the system's age. The apparent
relaxation time distribution extracted from the observed relaxations is thus
not "constant" but evolves with time. We discuss what defines the age of the
system and what external perturbation out of equilibrium does or does not
rejuvenate it. We further discuss the problem of relaxation times and comment
on the commonly used "two dip" experimental protocol aimed at extracting
"characteristic times" for the glassy systems (granular aluminum, doped indium
oxide...). We show that it is inoperable for systems like granular Al and
probably highly doped InOx where it provides a trivial value only determined by
the experimental protocol. But in cases where different values are obtained
like in lightly doped InOx or some ultra thin metal films, potentially
interesting information can be obtained, possibly about the "short time"
dynamics of the different systems. Present ideas about the effect of doping on
the glassiness of disordered insulators may also have to be reconsidered.Comment: to appear in the proceedings of the 14th International Conference on
Transport and Interactions in Disordered Systems (TIDS14
Aging phenomena in spin glass and ferromagnetic phases: domain growth and wall dynamics
We compare aging in a disordered ferromagnet and in a spin glass, by studying
the different phases of a reentrant system. We have measured the relaxation of
the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass
phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the
temperature is lowered (`chaos-like' effect) is observed in both phases. The
memory of previous aging at a higher temperature can be retrieved upon
re-heating, but in the ferromagnetic phase it can rapidly be erased by the
growth of ferromagnetic domains. We interpret the behaviour observed in the
ferromagnetic phase in terms of a combination of domain growth and pinned wall
reconformations, and suggest that aging in spin glasses is dominated by such
wall reconformation processes.Comment: SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France, to appear in
Europhys. Lett. (2000
Polymer Maximum Drag Reduction: A Unique Transitional State
The upper bound of polymer drag reduction is identified as a unique
transitional state between laminar and turbulent flow corresponding to the
onset of the nonlinear breakdown of flow instabilities
Multiple Signaling Functions Of Song In A Polymorphic Species With Alternative Reproductive Strategies
Vocal traits can be sexually selected to reflect male quality, but may also evolve to serve additional signaling functions. We used a long-term dataset to examine the signaling potential of song in dimorphic white-throated sparrows (Zonotrichia albicollis). We investigated whether song conveys multifaceted information about the vocalizing individual, including fitness, species identity, individual identity, and morph. We also evaluated whether song traits correlate differently with fitness in the two morphs, as the more promiscuous strategy of white, relative to tan, morph males might impose stronger sexual selection. Males with high song rates achieved higher lifetime reproductive success, and this pattern was driven by white morph males. In addition, males that sang songs with many notes survived longer, but this pattern was less robust. Thus, song traits reflect differences in fitness and may more strongly affect fitness in the white morph. Song frequency was unrelated to fitness, body size, or morph, but was individual specific and could signal individual identity. Songs of the two morphs displayed similar frequency ratios and bandwidths. However, tan morph males sang songs with longer first notes, fewer notes, and higher variability. Thus, song could be used in morph discrimination. Variation in frequency ratios between notes was low and could function in conspecific recognition, but pitch change dynamics did differ between four different song types observed. Our results support a multiple messages model for white-throated sparrow song, in which different song traits communicate discrete information about the vocalizing individual
Low-Temperature Features of Nano-Particle Dynamics
In view of better characterizing possible quantum effects in the dynamics of
nanometric particles, we measure the effect on the relaxation of a slight
heating cycle. The effect of the field amplitude is studied; its magnitude is
chosen in order to induce the relaxation of large particles (~7nm), even at
very low temperatures (100mK). Below 1K, the results significantly depart from
a simple thermal dynamics scenario.Comment: 1 tex file, 4 PostScript figure
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