1,934 research outputs found
Growing Cayley trees described by Fermi distribution
We introduce a model for growing Cayley trees with thermal noise. The
evolution of these hierarchical networks reduces to the Eden model and the
Invasion Percolation model in the limit , respectively.
We show that the distribution of the bond strengths (energies) is described by
the Fermi statistics. We discuss the relation of the present results with the
scale-free networks described by Bose statistics
Magnetorotational instability in cool cores of galaxy clusters
Clusters of galaxies are embedded in halos of optically thin, gravitationally
stratified, weakly magnetized plasma at the system's virial temperature. Due to
radiative cooling and anisotropic heat conduction, such intracluster medium
(ICM) is subject to local instabilities, which are combinations of the thermal,
magnetothermal and heat-flux-driven buoyancy instabilities. If the ICM rotates
significantly, its stability properties are substantially modified and, in
particular, also the magnetorotational instability (MRI) can play an important
role. We study simple models of rotating cool-core clusters and we demonstrate
that the MRI can be the dominant instability over significant portions of the
clusters, with possible implications for the dynamics and evolution of the cool
cores. Our results give further motivation for measuring the rotation of the
ICM with future X-ray missions such as ASTRO-H and ATHENA.Comment: 17 pages, 10 figures, accepted for publication in Journal of Plasma
Physics, Special Issue "Complex Plasma Phenomena in the Laboratory and in the
Universe
Effects of azimuth-symmetric acceptance cutoffs on the measured asymmetry in unpolarized Drell-Yan fixed target experiments
Fixed-target unpolarized Drell-Yan experiments often feature an acceptance
depending on the polar angle of the lepton tracks in the laboratory frame.
Typically leptons are detected in a defined angular range, with a dead zone in
the forward region. If the cutoffs imposed by the angular acceptance are
independent of the azimuth, at first sight they do not appear dangerous for a
measurement of the cos(2\phi)-asymmetry, relevant because of its association
with the violation of the Lam-Tung rule and with the Boer-Mulders function. On
the contrary, direct simulations show that up to 10 percent asymmetries are
produced by these cutoffs. These artificial asymmetries present qualitative
features that allow them to mimic the physical ones. They introduce some
model-dependence in the measurements of the cos(2\phi)-asymmetry, since a
precise reconstruction of the acceptance in the Collins-Soper frame requires a
Monte Carlo simulation, that in turn requires some detailed physical input to
generate event distributions. Although experiments in the eighties seem to have
been aware of this problem, the possibility of using the Boer-Mulders function
as an input parameter in the extraction of Transversity has much increased the
requirements of precision on this measurement. Our simulations show that the
safest approach to these measurements is a strong cutoff on the Collins-Soper
polar angle. This reduces statistics, but does not necessarily decrease the
precision in a measurement of the Boer-Mulders function.Comment: 13 pages, 14 figure
The gap amplification at a "shape resonance" in a superlattice of quantum stripes: a mechanism for high Tc
The amplification of the superconducting critical temperature Tc from the low
temperature range in homogeneous 2D planes (Tc<23 K) to the high temperature
range (23 K<Tc<150 K) in an artificial heterostructure of quantum stripes is
calculated. The high Tc is obtained by tuning the chemical potential near the
bottom of the nth subband at a "shape resonance", in a range, whithin the
energy cutoff for the pairing interaction. The resonance for the gap at the nth
"shape resonance" is studied for a free electron gas in the BCS approximation
as a function of the stripe width L, and of the number of electrons {\rho} per
unit surface. An amplification factor for coupling 0.1<{\lambda}<0.3 is
obtained at the third shape resonance raising the critical temperature in the
high Tc range.Comment: 9 pages 6 figure
Quantum statistics in complex networks
In this work we discuss the symmetric construction of bosonic and fermionic
networks and we present a case of a network showing a mixed quantum statistics.
This model takes into account the different nature of nodes, described by a
random parameter that we call energy, and includes rewiring of the links. The
system described by the mixed statistics is an inhomogemeous system formed by
two class of nodes. In fact there is a threshold energy such that
nodes with lower energy increase their connectivity
while nodes with higher energy decrease their
connectivity in time.Comment: 5 pages, 2 figure
Non-neutral theory of biodiversity
We present a non-neutral stochastic model for the dynamics taking place in a
meta-community ecosystems in presence of migration. The model provides a
framework for describing the emergence of multiple ecological scenarios and
behaves in two extreme limits either as the unified neutral theory of
biodiversity or as the Bak-Sneppen model. Interestingly, the model shows a
condensation phase transition where one species becomes the dominant one, the
diversity in the ecosystems is strongly reduced and the ecosystem is
non-stationary. This phase transition extend the principle of competitive
exclusion to open ecosystems and might be relevant for the study of the impact
of invasive species in native ecologies.Comment: 4 pages, 3 figur
A Superconductor Made by a Metal Heterostructure at the Atomic Limit Tuned at the "Shape Resonance": MgB2
We have studied the variation of Tc with charge density and lattice
parameters in Mg1-xAlxB2 superconducting samples at low Al doping x<8%. We show
that high Tc occurs where the chemical potential is tuned at a "superconducting
shape resonance" near the energy Ec of the quantum critical point (QCP) for the
dimensional transition from 2D to 3D electronic structure in a particular
subband of the natural superlattice of metallic atomic boron layers. At the
"shape resonance" the electrons pairs see a 2D Fermi surface at EF-w0 and a 3D
Fermi surface at EF+wo, where wo is the energy cut off of the pairing
interaction. The resonant amplification occurs in a narrow energy range where
EF-Ec is in the range of 2wo.Comment: 16 page
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