1,182 research outputs found
Emergence of Zipf's Law in the Evolution of Communication
Zipf's law seems to be ubiquitous in human languages and appears to be a
universal property of complex communicating systems. Following the early
proposal made by Zipf concerning the presence of a tension between the efforts
of speaker and hearer in a communication system, we introduce evolution by
means of a variational approach to the problem based on Kullback's Minimum
Discrimination of Information Principle. Therefore, using a formalism fully
embedded in the framework of information theory, we demonstrate that Zipf's law
is the only expected outcome of an evolving, communicative system under a
rigorous definition of the communicative tension described by Zipf.Comment: 7 pages, 2 figure
Robustness of the European power grids under intentional attack
The power grid defines one of the most important technological networks of
our times and sustains our complex society. It has evolved for more than a
century into an extremely huge and seemingly robust and well understood system.
But it becomes extremely fragile as well, when unexpected, usually minimal,
failures turn into unknown dynamical behaviours leading, for example, to sudden
and massive blackouts. Here we explore the fragility of the European power grid
under the effect of selective node removal. A mean field analysis of fragility
against attacks is presented together with the observed patterns. Deviations
from the theoretical conditions for network percolation (and fragmentation)
under attacks are analysed and correlated with non topological reliability
measures.Comment: 7 pages, 4 figure
Nonlinear Jaynes-Cummings model of atom-field interaction
Interaction of a two-level atom with a single mode of electromagnetic field
including Kerr nonlinearity for the field and intensity-dependent atom-field
coupling is discussed. The Hamiltonian for the atom-field system is written in
terms of the elements of a closed algebra, which has
SU(1,1) and Heisenberg-Weyl algebras as limiting cases. Eigenstates and
eigenvalues of the Hamiltonian are constructed.
With the field being in a coherent state initially, the dynamical behaviour
of atomic-inversion, field-statistics and uncertainties in the field
quadratures are studied. The appearance of nonclassical features during the
evolution of the field is shown. Further, we explore the overlap of initial and
time-evolved field states.Comment: 14 pages, 6 figures is PS forma
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Solid hydrogen structure
The J=0{minus}>2 Raman signal from solid J=0 D{sub 2} or H{sub 2} reveals HCP structure when deposited at a rate 0.1 {le} R({mu}/min) {le} 40 onto MgF{sub 2} at T{sub d}/T{sub tp} > 0.3, a mixture of HCP and FCC crystals at 0.2 < T{sub d}/T{sub tp} < 0.3 and possibly a randomly stacked close packed structure at T{sub d}/T{sub tp} < 0.2, where T{sub tp} is the triple point temperature. Non-HCP crystals transform to HCP continuously and irreversibly with increasing T. Finally, the crystal size decreases with decreasing T{sub d} and increasing R, from {approximately} 1 mm at T{sub d} {approximately} 0.8 T{sub tp} and R {approximately} 2 {mu}/min to {approximately} 1 {mu}m at 0.25 T{sub tp} and R {approximately} 40 {mu}/min
Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature
We present a new fabrication method of graphene spin-valve devices which
yields enhanced spin and charge transport properties by improving both the
electrode-to-graphene and graphene-to-substrate interface. First, we prepare
Co/MgO spin injection electrodes onto Si/SiO. Thereafter, we
mechanically transfer a graphene-hBN heterostructure onto the prepatterned
electrodes. We show that room temperature spin transport in single-, bi- and
trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion
lengths reaching 10m combined with carrier mobilities exceeding 20,000
cm/Vs.Comment: 15 pages, 5 figure
Topological reversibility and causality in feed-forward networks
Systems whose organization displays causal asymmetry constraints, from
evolutionary trees to river basins or transport networks, can be often
described in terms of directed paths (causal flows) on a discrete state space.
Such a set of paths defines a feed-forward, acyclic network. A key problem
associated with these systems involves characterizing their intrinsic degree of
path reversibility: given an end node in the graph, what is the uncertainty of
recovering the process backwards until the origin? Here we propose a novel
concept, \textit{topological reversibility}, which rigorously weigths such
uncertainty in path dependency quantified as the minimum amount of information
required to successfully revert a causal path. Within the proposed framework we
also analytically characterize limit cases for both topologically reversible
and maximally entropic structures. The relevance of these measures within the
context of evolutionary dynamics is highlighted.Comment: 9 pages, 3 figure
Expansion history and f(R) modified gravity
We attempt to fit cosmological data using modified Lagrangians
containing inverse powers of the Ricci scalar varied with respect to the
metric. While we can fit the supernova data well, we confirm the behaviour at medium to high redshifts reported elsewhere and argue
that the easiest way to show that this class of models are inconsistent with
the data is by considering the thickness of the last scattering surface. For
the best fit parameters to the supernova data, the simplest 1/R model gives
rise to a last scattering surface of thickness , inconsistent
with observations.Comment: accepted in JCAP, presentation clarified, results and conclusions
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