3,727 research outputs found
Logarithmic growth dynamics in software networks
In a recent paper, Krapivsky and Redner (Phys. Rev. E, 71 (2005) 036118)
proposed a new growing network model with new nodes being attached to a
randomly selected node, as well to all ancestors of the target node. The model
leads to a sparse graph with an average degree growing logarithmically with the
system size. Here we present compeling evidence for software networks being the
result of a similar class of growing dynamics. The predicted pattern of network
growth, as well as the stationary in- and out-degree distributions are
consistent with the model. Our results confirm the view of large-scale software
topology being generated through duplication-rewiring mechanisms. Implications
of these findings are outlined.Comment: 7 pages, 3 figures, published in Europhysics Letters (2005
Hole burning in a nanomechanical resonator coupled to a Cooper pair box
We propose a scheme to create holes in the statistical distribution of
excitations of a nanomechanical resonator. It employs a controllable coupling
between this system and a Cooper pair box. The success probability and the
fidelity are calculated and compared with those obtained in the atom-field
system via distinct schemes. As an application we show how to use the
hole-burning scheme to prepare (low excited) Fock states.Comment: 7 pages, 10 figure
Sustainable growth in complex networks
Based on the empirical analysis of the dependency network in 18 Java
projects, we develop a novel model of network growth which considers both: an
attachment mechanism and the addition of new nodes with a heterogeneous
distribution of their initial degree, . Empirically we find that the
cumulative degree distributions of initial degrees and of the final network,
follow power-law behaviors: , and
, respectively. For the total number of links as a
function of the network size, we find empirically ,
where is (at the beginning of the network evolution) between 1.25 and
2, while converging to for large . This indicates a transition from
a growth regime with increasing network density towards a sustainable regime,
which revents a collapse because of ever increasing dependencies. Our
theoretical framework is able to predict relations between the exponents
, , , which also link issues of software engineering and
developer activity. These relations are verified by means of computer
simulations and empirical investigations. They indicate that the growth of real
Open Source Software networks occurs on the edge between two regimes, which are
either dominated by the initial degree distribution of added nodes, or by the
preferential attachment mechanism. Hence, the heterogeneous degree distribution
of newly added nodes, found empirically, is essential to describe the laws of
sustainable growth in networks.Comment: 5 pages, 2 figures, 1 tabl
Two dimensional XXZ-Ising model on square-hexagon lattice
We study a two dimensional XXZ-Ising on square-hexagon (4-6) lattice with
spin-1/2. The phase diagram of the ground state energy is discussed, shown two
different ferrimagnetic states and two type of antiferromagnetic states, beside
of a ferromagnetic state. To solve this model, it could be mapped into the
eight-vertex model with union jack interaction term. Imposing exact solution
condition we find the region where the XXZ-Ising model on 4-6 lattice have
exact solutions with one free parameter, for symmetric eight-vertex model
condition. In this sense we explore the properties of the system and analyze
the competition of the interaction parameters providing the region where it has
an exact solution. However the present model does not satisfy the \textit{free
fermion} condition, unless for a trivial situation. Even so we are able to
discuss their critical points region, when the exactly solvable condition is
ignored.Comment: 5 pages, 5 figure
Production of Two Pions Induced by Neutrinos
We study the threshold production of two pions induced by neutrinos in
nucleon targets. The contribution of nucleon pole, pion and contact terms is
calculated using a chiral Lagrangian. The contribution of the Roper resonance,
neglected in earlier studies, has also been taken into account.Comment: Contribution to the proceedings of Chiral07, RCNP, Osaka, Japan.
Corrected version because of problems on some pdf viewer
Activity ageing in growing networks
We present a model for growing information networks where the ageing of a
node depends on the time at which it entered the network and on the last time
it was cited. The model is shown to undergo a transition from a small-world to
large-world network. The degree distribution may exhibit very different shapes
depending on the model parameters, e.g. delta-peaked, exponential or power-law
tailed distributions.Comment: 9 pages, 2 figure
Nuclear effects in neutrino induced reactions
We discuss the relevance of nuclear medium effects in the analysis of some
low and medium energy neutrino reactions of current interest. In particular, we
study the Quasi-Elastic (QE) process, where RPA correlations and Final State
Interactions (FSI) are shown to play a crucial role. We have also investigated
the neutrino induced coherent pion production. We find a strong reduction of
the cross section due to the distortion of the pion wave function and the
modification of the production mechanisms in the nucleus. The sensitivity of
the results to the axial coupling has been also
investigated.Comment: To appear in the proceedings of Chiral Symmetry in Hadron and Nuclear
Physics (Chiral07), Osaka, Japan, November 13-16, 200
Spatial chaos of an extensible conducting rod in a uniform magnetic field
The equilibrium equations for the isotropic Kirchhoff rod are known to form
an integrable system. It is also known that the effects of extensibility and
shearability of the rod do not break the integrable structure. Nor, as we have
shown in a previous paper does the effect of a magnetic field on a conducting
rod. Here we show, by means of Mel'nikov analysis, that, remarkably, the
combined effects do destroy integrability; that is, the governing equations for
an extensible current-carrying rod in a uniform magnetic field are
nonintegrable. This result has implications for possible configurations of
electrodynamic space tethers and may be relevant for electromechanical devices
Theoretical study of neutrino-induced coherent pion production off nuclei at T2K and MiniBooNE energies
We have developed a model for neutrino-induced coherent pion production off
nuclei in the energy regime of interest for present and forthcoming neutrino
oscillation experiments. It is based on a microscopic model for pion production
off the nucleon that, besides the dominant Delta pole contribution, takes into
account the effect of background terms required by chiral symmetry. Moreover,
the model uses a reduced nucleon-to-Delta resonance axial coupling, which leads
to coherent pion production cross sections around a factor two smaller than
most of the previous theoretical estimates. In the coherent production, the
main nuclear effects, namely medium corrections on the Delta propagator and the
final pion distortion, are included. We have improved on previous similar
models by taking into account the nucleon motion and employing a more
sophisticated optical potential. As found in previous calculations the
modification of the Delta self-energy inside the nuclear medium strongly
reduces the cross section, while the final pion distortion mainly shifts the
peak position to lower pion energies. The angular distribution profiles are not
much affected by nuclear effects. Nucleon motion increases the cross section by
15% at neutrino energies of 650 MeV, while Coulomb effects on charged pions are
estimated to be small. Finally, we discuss at length the deficiencies of the
Rein-Sehgal pion coherent production model for neutrino energies below 2 GeV,
and in particular for the MiniBooNE and T2K experiments. We also predict flux
averaged cross sections for these two latter experiments and K2K.Comment: 19 latex pages, 10 figures, 2 tables. Minor changes. Version accepted
for publication in Physical Review
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