3,787 research outputs found
Critical behavior of an Ising model with aperiodic interactions
We write exact renormalization-group recursion relations for a ferromagnetic
Ising model on the diamond hierarchical lattice with an aperiodic distribution
of exchange interactions according to a class of generalized two-letter
Fibonacci sequences. For small geometric fluctuations, the critical behavior is
unchanged with respect to the uniform case. For large fluctuations, the uniform
fixed point in the parameter space becomes fully unstable. We analyze some
limiting cases, and propose a heuristic criterion to check the relevance of the
fluctuations.Comment: latex file, 5 figures, accepted by Braz. Jour. Phy
The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796
The isolated elliptical (IE) NGC 7796 is accompanied by an interesting
early-type dwarf galaxy, named NGC7796-DW1. It exhibits a tidal tail, very boxy
isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the
bulk population of the galaxy, indicating a younger age. These properties are
suggestive of a dwarf-dwarf merger remnant. We use the Multi-Unit Spectroscopic
Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract
characteristic spectra to which we apply the STARLIGHT population synthesis
software to obtain ages and metallicities of the various population components
of the galaxy. The galaxy's main body is old and metal-poor. A surprising
result is the extended line emission in the galaxy, forming a ring-like
structure with a projected diameter of 2.2 kpc. The line ratios fall into the
regime of HII-regions, although OB-stellar populations cannot be identified by
spectral signatures. Nucleus A is a relatively old (7 Gyr or older) and
metal-poor super star cluster, most probably the nucleus of the dwarf, now
displaced. The star-forming regions B and C show younger and distinctly more
metal-rich components. The emission line ratios of regions B and C indicate an
almost solar oxygen abundance, if compared with radiation models of HII
regions. NGC7796-DW1 occupies a particular role in the group of transition-type
galaxies with respect to its origin and current evolutionary state, being the
companion of an IE. The dwarf-dwarf merger scenario is excluded because of the
missing metal-rich merger component. A viable alternative is gas accretion from
a reservoir of cold, metal-rich gas. NGC7796 has to provide this gas within its
X-ray bright halo. As illustrated by NGC7796-DW1, cold accretion may be a
general solution to the problem of extended star formation histories in
transition dwarf galaxies. (abridged)Comment: comments: 13 pages, 8 figures, accepted for publication in Astronomy
& Astrophysic
The Moments of the Hydrogen Atom by the Method of Brackets
Expectation values of powers of the radial coordinate in arbitrary hydrogen
states are given, in the quantum case, by an integral involving the associated
Laguerre function. The method of brackets is used to evaluate the integral in
closed-form and to produce an expression for this average value as a finite
sum
The fluctuation-dissipation theorem and the linear Glauber model
We obtain exact expressions for the two-time autocorrelation and response
functions of the -dimensional linear Glauber model. Although this linear
model does not obey detailed balance in dimensions , we show that the
usual form of the fluctuation-dissipation ratio still holds in the stationary
regime. In the transient regime, we show the occurence of aging, with a special
limit of the fluctuation-dissipation ratio, , for a quench at
the critical point.Comment: Accepted for publication (Physical Review E
A thermodynamical fiber bundle model for the fracture of disordered materials
We investigate a disordered version of a thermodynamic fiber bundle model
proposed by Selinger, Wang, Gelbart, and Ben-Shaul a few years ago. For simple
forms of disorder, the model is analytically tractable and displays some new
features. At either constant stress or constant strain, there is a non
monotonic increase of the fraction of broken fibers as a function of
temperature. Moreover, the same values of some macroscopic quantities as stress
and strain may correspond to different microscopic cofigurations, which can be
essential for determining the thermal activation time of the fracture. We argue
that different microscopic states may be characterized by an experimentally
accessible analog of the Edwards-Anderson parameter. At zero temperature, we
recover the behavior of the irreversible fiber bundle model.Comment: 18 pages, 10 figure
Courageous Conversations: Advising the Foreclosed Student
Institutions with restricted enrollment programs share the common predicament of working with students who have steadfastly dedicated themselves to the goal of admission to one program and no other. For these students, those who are identified as foreclosed through their attachment to one major without consideration of other options, the admission/denial situation becomes an endless cycle of frustration and possible attrition. GPA requirements, global competition for placement, frequent admission criteria changes, and a confusing array of requirements across institutions all contribute to the stress felt by these students, parents, support team members, and most certainly, advisors. This article offers an approach for advisors to engage students in a broader and improved awareness of options available when students face denial from their intended major
Field behavior of an Ising model with aperiodic interactions
We derive exact renormalization-group recursion relations for an Ising model,
in the presence of external fields, with ferromagnetic nearest-neighbor
interactions on Migdal-Kadanoff hierarchical lattices. We consider layered
distributions of aperiodic exchange interactions, according to a class of
two-letter substitutional sequences. For irrelevant geometric fluctuations, the
recursion relations in parameter space display a nontrivial uniform fixed point
of hyperbolic character that governs the universal critical behavior. For
relevant fluctuations, in agreement with previous work, this fixed point
becomes fully unstable, and there appears a two-cycle attractor associated with
a new critical universality class.Comment: 9 pages, 1 figure (included). Accepted for publication in Int. J.
Mod. Phys.
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