32,130 research outputs found
Aspects of reduction and transformation of Lagrangian systems with symmetry
This paper contains results on geometric Routh reduction and it is a
continuation of a previous paper where a new class of transformations is
introduced between Lagrangian systems obtained after Routh reduction. In
general, these reduced Lagrangian systems have magnetic force terms and are
singular in the sense that the Lagrangian does not depend on some velocity
components. The main purpose of this paper is to show that the Routh reduction
process itself is entirely captured by the application of such a new
transformation on the initial Lagrangian system with symmetry.Comment: To appear in Journal of Geometric Mechanic
Anderson transition in systems with chiral symmetry
Anderson localization is a universal quantum feature caused by destructive
interference. On the other hand chiral symmetry is a key ingredient in
different problems of theoretical physics: from nonperturbative QCD to highly
doped semiconductors. We investigate the interplay of these two phenomena in
the context of a three-dimensional disordered system. We show that chiral
symmetry induces an Anderson transition (AT) in the region close to the band
center. Typical properties at the AT such as multifractality and critical
statistics are quantitatively affected by this additional symmetry. The origin
of the AT has been traced back to the power-law decay of the eigenstates; this
feature may also be relevant in systems without chiral symmetry.Comment: RevTex4, 4 two-column pages, 3 .eps figures, updated references,
final version as published in Phys. Rev.
Episodic synchronization in dynamically driven neurons
We examine the response of type II excitable neurons to trains of synaptic
pulses, as a function of the pulse frequency and amplitude. We show that the
resonant behavior characteristic of type II excitability, already described for
harmonic inputs, is also present for pulsed inputs. With this in mind, we study
the response of neurons to pulsed input trains whose frequency varies
continuously in time, and observe that the receiving neuron synchronizes
episodically to the input pulses, whenever the pulse frequency lies within the
neuron's locking range. We propose this behavior as a mechanism of rate-code
detection in neuronal populations. The results are obtained both in numerical
simulations of the Morris-Lecar model and in an electronic implementation of
the FitzHugh-Nagumo system, evidencing the robustness of the phenomenon.Comment: 7 pages, 8 figure
A Catalogue of Be Stars in the Direction of the Galactic Bulge
Detailed studies of Be stars in environments with different metallicities
like the Magellanic Clouds or the Galactic bulge are necessary to understand
the formation and evolution mechanisms of the circumstellar disks. However, a
detailed study of Be stars in the direction of the bulge of our own galaxy has
not been performed until now. We report the first systematic search for Be star
candidates in the direction of the Galactic Bulge. We present the catalogue,
give a brief description of the stellar variability seen, and show some light
curve examples. We searched for stars matching specific criteria of magnitude,
color and variability in the I band. Our search was conducted on the 48 OGLE II
fields of the Galactic Bulge.This search has resulted in 29053 Be star
candidates, 198 of them showing periodic light variations. Nearly 1500 stars in
this final sample are almost certainly Be stars, providing an ideal sample for
spectroscopic multiobject follow-up studies.Comment: Accepted for publication in A &
Routh reduction and the class of magnetic Lagrangian systems
In this paper, some new aspects related to Routh reduction of Lagrangian
systems with symmetry are discussed. The main result of this paper is the
introduction of a new concept of transformation that is applicable to systems
obtained after Routh reduction of Lagrangian systems with symmetry, so-called
magnetic Lagrangian systems. We use these transformations in order to show
that, under suitable conditions, the reduction with respect to a (full)
semi-direct product group is equivalent to the reduction with respect to an
Abelian normal subgroup. The results in this paper are closely related to the
more general theory of Routh reduction by stages.Comment: 23 page
Klein tunneling and Dirac potentials in trapped ions
We propose the quantum simulation of the Dirac equation with potentials,
allowing the study of relativistic scaterring and the Klein tunneling. This
quantum relativistic effect permits a positive-energy Dirac particle to
propagate through a repulsive potential via the population transfer to
negative-energy components. We show how to engineer scalar, pseudoscalar, and
other potentials in the 1+1 Dirac equation by manipulating two trapped ions.
The Dirac spinor is represented by the internal states of one ion, while its
position and momentum are described by those of a collective motional mode. The
second ion is used to build the desired potentials with high spatial
resolution.Comment: 4 pages, 3 figures, minor change
Entropy and Entanglement in Quantum Ground States
We consider the relationship between correlations and entanglement in gapped
quantum systems, with application to matrix product state representations. We
prove that there exist gapped one-dimensional local Hamiltonians such that the
entropy is exponentially large in the correlation length, and we present strong
evidence supporting a conjecture that there exist such systems with arbitrarily
large entropy. However, we then show that, under an assumption on the density
of states which is believed to be satisfied by many physical systems such as
the fractional quantum Hall effect, that an efficient matrix product state
representation of the ground state exists in any dimension. Finally, we comment
on the implications for numerical simulation.Comment: 7 pages, no figure
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