46,439 research outputs found
Elementary transitions and magnetic correlations in two-dimensional disordered nanoparticle ensembles
The magnetic relaxation processes in disordered two-dimensional ensembles of
dipole-coupled magnetic nanoparticles are theoretically investigated by
performing numerical simulations. The energy landscape of the system is
explored by determining saddle points, adjacent local minima, energy barriers,
and the associated minimum energy paths (MEPs) as functions of the structural
disorder and particle density. The changes in the magnetic order of the
nanostructure along the MEPs connecting adjacent minima are analyzed from a
local perspective. In particular, we determine the extension of the correlated
region where the directions of the particle magnetic moments vary
significantly. It is shown that with increasing degree of disorder the magnetic
correlation range decreases, i.e., the elementary relaxation processes become
more localized. The distribution of the energy barriers, and their relation to
the changes in the magnetic configurations are quantified. Finally, some
implications for the long-time magnetic relaxation dynamics of nanostructures
are discussed.Comment: 19 pages, 6 figure
Fear and its implications for stock markets
The value of stocks, indices and other assets, are examples of stochastic
processes with unpredictable dynamics. In this paper, we discuss asymmetries in
short term price movements that can not be associated with a long term positive
trend. These empirical asymmetries predict that stock index drops are more
common on a relatively short time scale than the corresponding raises. We
present several empirical examples of such asymmetries. Furthermore, a simple
model featuring occasional short periods of synchronized dropping prices for
all stocks constituting the index is introduced with the aim of explaining
these facts. The collective negative price movements are imagined triggered by
external factors in our society, as well as internal to the economy, that
create fear of the future among investors. This is parameterized by a ``fear
factor'' defining the frequency of synchronized events. It is demonstrated that
such a simple fear factor model can reproduce several empirical facts
concerning index asymmetries. It is also pointed out that in its simplest form,
the model has certain shortcomings.Comment: 5 pages, 5 figures. Submitted to the Proceedings of Applications of
Physics in Financial Analysis 5, Turin 200
Arrays of Josephson junctions in an environment with vanishing impedance
The Hamiltonian operator for an unbiased array of Josephson junctions with
gate voltages is constructed when only Cooper pair tunnelling and charging
effects are taken into account. The supercurrent through the system and the
pumped current induced by changing the gate voltages periodically are discussed
with an emphasis on the inaccuracies in the Cooper pair pumping.
Renormalisation of the Hamiltonian operator is used in order to reliably
parametrise the effects due to inhomogeneity in the array and non-ideal gating
sequences. The relatively simple model yields an explicit, testable prediction
based on three experimentally motivated and determinable parameters.Comment: 13 pages, 9 figures, uses RevTeX and epsfig, Revised version, Better
readability and some new result
1/z-renormalization of the mean-field behavior of the dipole-coupled singlet-singlet system HoF_3
The two main characteristics of the holmium ions in HoF_3 are that their
local electronic properties are dominated by two singlet states lying well
below the remaining 4f-levels, and that the classical dipole-coupling is an
order of magnitude larger than any other two-ion interactions between the
Ho-moments. This combination makes the system particularly suitable for testing
refinements of the mean-field theory. There are four Ho-ions per unit cell and
the hyperfine coupled electronic and nuclear moments on the Ho-ions order in a
ferrimagnetic structure at T_C=0.53 K. The corrections to the mean-field
behavior of holmium triflouride, both in the paramagnetic and ferrimagnetic
phase, have been calculated to first order in the high-density 1/z-expansion.
The effective medium theory, which includes the effects of the single-site
fluctuations, leads to a substantially improved description of the magnetic
properties of HoF_3, in comparison with that based on the mean-field
approximation.Comment: 26pp, plain-TeX, JJ
Dimensional reduction in a model with infinitely many absorbing states
Using Monte Carlo method we study a two-dimensional model with infinitely
many absorbing states. Our estimation of the critical exponent beta=0.273(5)
suggests that the model belongs to the (1+1) rather than (2+1)
directed-percolation universality class. We also show that for a large class of
absorbing states the dynamic Monte Carlo method leads to spurious dynamical
transitions.Comment: 6 pages, 4 figures, Phys.Rev. E, Dec. 199
Synthesis of imide/arylene ether copolymers for adhesives and composite matrices
A series of imide/arylene ether copolymers were prepared from the reaction of an amorphous arylene ether oligomer and a semi-crystalline imide oligomer. These copolymers were thermally characterized and mechanical properties were measured. One block copolymer was endcapped and the molecular weight was controlled to provide a material that displayed good compression moldability and attractive adhesion and composite properties
Reconsidering "the love of art" : evaluating the potential of art museum outreach
Art museums have long been identified as bastions of social and cultural exclusion. This conclusion was best evidenced by the large-scale 1967 French study by Bourdieu and Darbel demonstrating the exclusionary nature of “The Love of Art.” However, in recent years there have been increasing efforts to reach out to a broader range of visitors beyond conventional
audiences. The present study investigates the impacts of an outreach program at a UK art museum, which sought to engage socially excluded young mothers. This study employs ethnographic research methods on a longitudinal basis to develop qualitative insights about the program seeking to mitigate cultural exclusion. While the study’s findings uphold many longstanding critiques of art museums’ conventional approaches, the study also indicates that carefully designed outreach activities can overcome such limitations and enhance cultural
engagement. Thus, art museums’ limited appeal is tied to problematic public engagement practices that can be changed
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