269 research outputs found
Synergetic Analysis of the Haeussler-von der Malsburg Equations for Manifolds of Arbitrary Geometry
We generalize a model of Haeussler and von der Malsburg which describes the
self-organized generation of retinotopic projections between two
one-dimensional discrete cell arrays on the basis of cooperative and
competitive interactions of the individual synaptic contacts. Our generalized
model is independent of the special geometry of the cell arrays and describes
the temporal evolution of the connection weights between cells on different
manifolds. By linearizing the equations of evolution around the stationary
uniform state we determine the critical global growth rate for synapses onto
the tectum where an instability arises. Within a nonlinear analysis we use then
the methods of synergetics to adiabatically eliminate the stable modes near the
instability. The resulting order parameter equations describe the emergence of
retinotopic projections from initially undifferentiated mappings independent of
dimension and geometry.Comment: Dedicated to Hermann Haken on the occasion of his 80th birthda
Disorder Effects in Superconducting Multiple Loop Quantum Interferometers
A theoretical study is presented on a number N of resistively shunted
Josephson junctions connected in parallel as a disordered 1D array by
superconducting wiring in such a manner that there are N-1 individual SQUID
loops with arbitrary shape formed.Comment: 4 pages, 2 figure
Classification of one-dimensional quasilattices into mutual local-derivability classes
One-dimensional quasilattices are classified into mutual local-derivability
(MLD) classes on the basis of geometrical and number-theoretical
considerations. Most quasilattices are ternary, and there exist an infinite
number of MLD classes. Every MLD class has a finite number of quasilattices
with inflation symmetries. We can choose one of them as the representative of
the MLD class, and other members are given as decorations of the
representative. Several MLD classes of particular importance are listed. The
symmetry-preserving decorations rules are investigated extensively.Comment: 42 pages, latex, 5 eps figures, Published in JPS
Mesoscopic Aharonov-Bohm oscillations in metallic rings
We study the amplitude of mesoscopic Aharonov-Bohm oscillations in
quasi-one-dimensional (Q1D) diffusive rings. We consider first the
low-temperature limit of a fully coherent sample. The variance of oscillation
harmonics is calculated as a function of the length of the leads attaching the
ring to reservoirs. We further analyze the regime of relatively high
temperatures, when the dephasing due to electron-electron interaction
suppresses substantially the oscillations. We show that the dephasing length
L_phi^AB governing the damping factor exp(-2pi R /L_phi^AB) of the oscillations
is parametrically different from the common dephasing length for the Q1D
geometry. This is due to the fact that the dephasing is governed by energy
transfers determined by the ring circumference 2pi R, making L_phi^AB
R-dependent.Comment: 16 pages, 4 figures, to appear in proceedings of NATO/Euresco
Conference "Fundamental Problems of Mesoscopic Physics: Interactions and
Decoherence", Granada (Spain), September 200
Non-equilibrium electronic transport and interaction in short metallic nanobridges
We have observed interaction effects in the differential conductance of
short, disordered metal bridges in a well-controlled non-equilibrium situation,
where the distribution function has a double Fermi step. A logarithmic scaling
law is found both for the temperature and for the voltage dependence of in
all samples. The absence of magnetic field dependence and the low
dimensionality of our samples allow us to distinguish between several possible
interaction effects, proposed recently in nanoscopic samples. The universal
scaling curve is explained quantitatively by the theory of electron-electron
interaction in diffusive metals, adapted to the present case, where the sample
size is smaller than the thermal diffusion length.Comment: Published version, 6 Pages, 6 postscript figures, 1 tabl
Antiferromagnetic interlayer exchange coupling across an amorphous metallic spacer layer
By means of magneto-optical Kerr effect we observe for the first time
antiferromagnetic coupling between ferromagnetic layers across an amorphous
metallic spacer layer. Biquadratic coupling occurs at the transition from a
ferromagnetically to an antiferromagnetically coupled region. Scanning
tunneling microscopy images of all involved layers are used to extract
thickness fluctuations and to verify the amorphous state of the spacer. The
observed antiferromagnetic coupling behavior is explained by RKKY interaction
taking into account the amorphous structure of the spacer material.Comment: Typset using RevTex, 4 pages with 4 figures (.eps
Bar Evolution Over the Last Eight Billion Years: A Constant Fraction of Strong Bars in GEMS
One third of present-day spirals host optically visible strong bars that
drive their dynamical evolution. However, the fundamental question of how bars
evolve over cosmological times has yet to be addressed, and even the frequency
of bars at intermediate redshifts remains controversial. We investigate the
frequency of bars out to z~1.0 drawing on a sample of 1590 galaxies from the
GEMS survey, which provides morphologies from HST ACS two-color images, and
highly accurate redshifts from the COMBO-17 survey. We identify spiral galaxies
using the Sersic index, concentration parameter, and rest-frame color. We
characterize bars and disks by fitting ellipses to F606W and F850LP images,
taking advantage of the two bands to minimize bandpass shifting. We exclude
highly inclined (i>60 deg) galaxies to ensure reliable morphological
classifications, and apply completeness cuts of M_v <= -19.3 and -20.6. More
than 40% of the bars that we detect have semi major axes a<0.5" and would be
easily missed in earlier surveys without the small PSF of ACS. The bars that we
can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in
the range ~1.2-13 kpc. We find that the optical fraction of such strong bars
remains at ~(30% +- 6%) from the present-day out to look-back times of 2-6 Gyr
(z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0); it certainly shows no sign of a drastic
decline at z>0.7. Our findings of a large and similar bar fraction at these
three epochs favor scenarios in which cold gravitationally unstable disks are
already in place by z~1, and where on average bars have a long lifetime (well
above 2 Gyr). The distributions of structural bar properties in the two slices
are, however, not statistically identical and therefore allow for the
possibility that the bar strengths and sizes may evolve over time.Comment: Accepted by ApJ Letters, to appear in Nov 2004 issue. Minor
revisions,updated reference
Cluster Performance reconsidered: Structure, Linkages and Paths in the German Biotechnology Industry, 1996-2003
This paper addresses the evolution of biotechnology clusters in Germany between 1996 and 2003, paying particular attention to their respective composition in terms of venture capital, basic science institutions and biotechnology firms. Drawing upon the significance of co-location of "money and ideas", the literature stressing the importance of a cluster's openness and external linkages, and the path dependency debate, the paper aims to analyse how certain cluster characteristics correspond with its overall performance. After identifying different cluster types, we investigate their internal and external interconnectivity in comparative manner and draw on changes in cluster composition. Our results indicate that the structure, i.e. to which group the cluster belongs, and the openness towards external knowledge flows deliver merely unsystematic indications with regard to a cluster's overall success. Its ability to change composition towards a more balanced ratio of science and capital over time, on the other hand, turns out as a key explanatory factor. Hence, the dynamic perspective proves effective illuminating cluster growth and performance, where our explorative findings provide a promising avenue for further evolutionary research
CANDELS Observations of the Structural Properties and Evolution of Galaxies in a Cluster at z=1.62
We discuss the structural and morphological properties of galaxies in a
z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope
Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology
relation: galaxies with colors of quiescent stellar populations generally have
morphologies consistent with spheroids, and galaxies with colors consistent
with ongoing star formation have disk-like and irregular morphologies. The size
distribution of the quiescent cluster galaxies shows a deficit of compact (<
1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a
result the cluster quiescent galaxies have larger average effective sizes
compared to field galaxies at fixed mass at greater than 90% significance.
Combined with data from the literature, the size evolution of quiescent cluster
galaxies is relatively slow from z~1.6 to the present, growing as
(1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that
physical processes associated with the denser cluster region seems to have
caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower
subsequent growth at z<1.6 compared to galaxies in the lower density field. The
quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower
redshift samples at fixed mass, and their surface-brightness profiles suggest
that they contain extended stellar disks. We argue the cluster galaxies require
dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk
material and to match the relations for ellipticity, stellar mass, size, and
color of early-type galaxies in z<1 clusters.Comment: Accepted for publication in ApJ. 14 pages in emulateapj format.
Replacement includes improvements from referee report, and updates and
additions to reference
Detailed Decomposition of Galaxy Images. II. Beyond Axisymmetric Models
We present a two-dimensional (2-D) fitting algorithm (GALFIT, Version 3) with
new capabilities to study the structural components of galaxies and other
astronomical objects in digital images. Our technique improves on previous 2-D
fitting algorithms by allowing for irregular, curved, logarithmic and power-law
spirals, ring and truncated shapes in otherwise traditional parametric
functions like the Sersic, Moffat, King, Ferrer, etc., profiles. One can mix
and match these new shape features freely, with or without constraints, apply
them to an arbitrary number of model components and of numerous profile types,
so as to produce realistic-looking galaxy model images. Yet, despite the
potential for extreme complexity, the meaning of the key parameters like the
Sersic index, effective radius or luminosity remain intuitive and essentially
unchanged. The new features have an interesting potential for use to quantify
the degree of asymmetry of galaxies, to quantify low surface brightness tidal
features beneath and beyond luminous galaxies, to allow more realistic
decompositions of galaxy subcomponents in the presence of strong rings and
spiral arms, and to enable ways to gauge the uncertainties when decomposing
galaxy subcomponents. We illustrate these new features by way of several case
studies that display various levels of complexity.Comment: 41 pages, 22 figures, AJ accepted. Minor changes. Full resolution
version of this paper is available at:
http://users.obs.carnegiescience.edu/peng/work/galfit/galfit3.pd
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