269 research outputs found
Langevin Simulations of Two Dimensional Vortex Fluctuations: Anomalous Dynamics and a New -exponent
The dynamics of two dimensional (2D) vortex fluctuations are investigated
through simulations of the 2D Coulomb gas model in which vortices are
represented by soft disks with logarithmic interactions. The simulations
trongly support a recent suggestion that 2D vortex fluctuations obey an
intrinsic anomalous dynamics manifested in a long range 1/t-tail in the vortex
correlations. A new non-linear IV-exponent a, which is different from the
commonly used AHNS exponent, a_AHNS and is given by a = 2a_AHNS - 3, is
confirmed by the simulations. The results are discussed in the context of
earlier simulations, experiments and a phenomenological description.Comment: Submitted to PRB, RevTeX format, 28 pages and 13 figures, figures in
postscript format are available at http://www.tp.umu.se/~holmlund/papers.htm
Navigating Networks with Limited Information
We study navigation with limited information in networks and demonstrate that
many real-world networks have a structure which can be described as favoring
communication at short distance at the cost of constraining communication at
long distance. This feature, which is robust and more evident with limited than
with complete information, reflects both topological and possibly functional
design characteristics. For example, the characteristics of the networks
studied derived from a city and from the Internet are manifested through
modular network designs. We also observe that directed navigation in typical
networks requires remarkably little information on the level of individual
nodes. By studying navigation, or specific signaling, we take a complementary
approach to the common studies of information transfer devoted to broadcasting
of information in studies of virus spreading and the like.Comment: 6 pages, 6 figures. For associated Java applet, see
http://cmol.nbi.dk/models/bit/bit.htm
Networks and Cities: An Information Perspective
Traffic is constrained by the information involved in locating the receiver
and the physical distance between sender and receiver. We here focus on the
former, and investigate traffic in the perspective of information handling. We
re-plot the road map of cities in terms of the information needed to locate
specific addresses and create information city networks with roads mapped to
nodes and intersections to links between nodes. These networks have the broad
degree distribution found in many other complex networks. The mapping to an
information city network makes it possible to quantify the information
associated with locating specific addresses.Comment: 4 pages, 4 figure
Searchability of Networks
We investigate the searchability of complex systems in terms of their
interconnectedness. Associating searchability with the number and size of
branch points along the paths between the nodes, we find that scale-free
networks are relatively difficult to search, and thus that the abundance of
scale-free networks in nature and society may reflect an attempt to protect
local areas in a highly interconnected network from nonrelated communication.
In fact, starting from a random node, real-world networks with higher order
organization like modular or hierarchical structure are even more difficult to
navigate than random scale-free networks. The searchability at the node level
opens the possibility for a generalized hierarchy measure that captures both
the hierarchy in the usual terms of trees as in military structures, and the
intrinsic hierarchical nature of topological hierarchies for scale-free
networks as in the Internet.Comment: 9 pages, 10 figure
Resistance scaling at the Kosterlitz-Thouless transition
We study the linear resistance at the Kosterlitz-Thouless transition by Monte
Carlo simulation of vortex dynamics. Finite size scaling analysis of our data
show excellent agreement with scaling properties of the Kosterlitz-Thouless
transition. We also compare our results for the linear resistance with
experiments. By adjusting the vortex chemical potential to an optimum value,
the resistance at temperatures above the transition temperature agrees well
with experiments over many decades.Comment: 7 pages, 4 postscript figures included, LATEX, KTH-CMT-94-00
Scale-freeness for networks as a degenerate ground state: A Hamiltonian formulation
The origin of scale-free degree distributions in the context of networks is
addressed through an analogous non-network model in which the node degree
corresponds to the number of balls in a box and the rewiring of links to balls
moving between the boxes. A statistical mechanical formulation is presented and
the corresponding Hamiltonian is derived. The energy, the entropy, as well as
the degree distribution and its fluctuations are investigated at various
temperatures. The scale-free distribution is shown to correspond to the
degenerate ground state, which has small fluctuations in the degree
distribution and yet a large entropy. We suggest an implication of our results
from the viewpoint of the stability in evolution of networks.Comment: 7 pages, 3 figures. To appear in Europhysics lette
Phase Diagram of the Two Dimensional Lattice Coulomb Gas
We use Monte Carlo simulations to map out the phase diagram of the two
dimensional Coulomb gas on a square lattice, as a function of density and
temperature. We find that the Kosterlitz-Thouless transition remains up to
higher charge densities than has been suggested by recent theoretical
estimates.Comment: 4 pages, including 6 in-line eps figure
New Results on the Phase Diagram of the FFXY Model: A Twisted CFT Approach
The issue of the number, nature and sequence of phase transitions in the
fully frustrated XY (FFXY) model is a highly non trivial one due to the complex
interplay between its continuous and discrete degrees of freedom. In this
contribution we attack such a problem by means of a twisted conformal field
theory (CFT) approach and show how it gives rise to the U (1)
symmetry and to the whole spectrum of excitations of the FFXY model.Comment: 7 pages; talk given by G. Niccoli at "Path Integrals - New Trends and
Perspectives International Conference", Max-Planck-Institut, Dresden,
Germany, September 23 - 28, 200
Transverse voltage in high-Tc superconductors in zero magnetic fields
Longitudinal and transverse voltages have been measured in zero external
magnetic fields. In close vicinity of the superconducting transition nonzero
transverse voltage has been observed while far away from Tc, both above and
below no such voltage has been detected. The value of the transverse
resistivity depends on the value of the transport current. Several models have
been discussed taking into account also the penetration of self field due to
the applied transport current. It seems that observed results can be explained
using the Kosterlitz-Thouless model as a result of an unpairing of
vortex-antivortex pairs created below Tkt due to fluctuations. At Tkt free
vortices and antivortices are created and can contribute to a dissipation of
energy. Their movement should also be responsible for the observed nonzero
transverse voltage.Comment: 3 pages in Latex, 3 figs.ep
Neutral theory of chemical reaction networks
To what extent do the characteristic features of a chemical reaction network
reflect its purpose and function? In general, one argues that correlations
between specific features and specific functions are key to understanding a
complex structure. However, specific features may sometimes be neutral and
uncorrelated with any system-specific purpose, function or causal chain. Such
neutral features are caused by chance and randomness. Here we compare two
classes of chemical networks: one that has been subjected to biological
evolution (the chemical reaction network of metabolism in living cells) and one
that has not (the atmospheric planetary chemical reaction networks). Their
degree distributions are shown to share the very same neutral
system-independent features. The shape of the broad distributions is to a large
extent controlled by a single parameter, the network size. From this
perspective, there is little difference between atmospheric and metabolic
networks; they are just different sizes of the same random assembling network.
In other words, the shape of the degree distribution is a neutral
characteristic feature and has no functional or evolutionary implications in
itself; it is not a matter of life and death.Comment: 13 pages, 8 figure
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