18,484 research outputs found
Landau degeneracy and black hole entropy
We consider the supergravity solution describing a configuration of
intersecting D-4-branes with non-vanishing worldvolume gauge fields. The
entropy of such a black hole is calculated in terms of the D-branes quantised
charges. The non-extreme solution is also considered and the corresponding
thermodynamical quantities are calculated in terms of a D-brane/anti-D-brane
system. To perform the quantum mechanical D-brane analysis we study
open-strings with their ends on branes with a magnetic condensate. Applying the
results to our D-brane system we managed to have a perfect agreement between
the D-brane entropy counting and the corresponding semi-classical result. The
Landau degeneracy of the open string states describing the excitations of the
D-brane system enters in a crucial way. We also derive the near-extreme results
which agree with the semi-classical calculations.Comment: 30 pages, 1 figure, latex. Minor corrections, version to appear in
Nuclear Physics
Activation of Non-Local Quantum Resources
We find two two-qubit states such that any number of copies of one state or
the other cannot violate the CHSH Bell inequality. However, their tensor
product can produce a CHSH violation of at least 2.023. We also identify a
CHSH-local state such that two copies of it are CHSH-violating. The tools
employed here can be easily adapted to find instances of non-locality
activation in arbitrary Bell scenarios
Integrability, Supersymmetry and Coherent States. A volume in honour of Professor VĂ©ronique Hussin
ISBN 978-3-030-20087-
Opinion dynamics driven by leaders, media, viruses and worms
A model on the effects of leader, media, viruses, and worms and other agents
on the opinion of individuals is developed and utilized to simulate the
formation of consensus in society and price in market via excess between supply
and demand. Effects of some time varying drives, (harmonic and hyperbolic) are
also investigated.
Key words: Opinion; Leader; Media; Market; Buyers; Sellers; ExcessComment: 14 pages, 7 figures (14, total) Will be published in IJMP
The warp drive: hyper-fast travel within general relativity
It is shown how, within the framework of general relativity and without the
introduction of wormholes, it is possible to modify a spacetime in a way that
allows a spaceship to travel with an arbitrarily large speed. By a purely local
expansion of spacetime behind the spaceship and an opposite contraction in
front of it, motion faster than the speed of light as seen by observers outside
the disturbed region is possible. The resulting distortion is reminiscent of
the ``warp drive'' of science fiction. However, just as it happens with
wormholes, exotic matter will be needed in order to generate a distortion of
spacetime like the one discussed here.Comment: 10 pages, 1 figure. Not previously available in gr-q
The entropic origin of disassortativity in complex networks
Why are most empirical networks, with the prominent exception of social ones,
generically degree-degree anticorrelated, i.e. disassortative? With a view to
answering this long-standing question, we define a general class of
degree-degree correlated networks and obtain the associated Shannon entropy as
a function of parameters. It turns out that the maximum entropy does not
typically correspond to uncorrelated networks, but to either assortative
(correlated) or disassortative (anticorrelated) ones. More specifically, for
highly heterogeneous (scale-free) networks, the maximum entropy principle
usually leads to disassortativity, providing a parsimonious explanation to the
question above. Furthermore, by comparing the correlations measured in some
real-world networks with those yielding maximum entropy for the same degree
sequence, we find a remarkable agreement in various cases. Our approach
provides a neutral model from which, in the absence of further knowledge
regarding network evolution, one can obtain the expected value of correlations.
In cases in which empirical observations deviate from the neutral predictions
-- as happens in social networks -- one can then infer that there are specific
correlating mechanisms at work.Comment: 4 pages, 4 figures. Accepted in Phys. Rev. Lett. (2010
Neighborhood models of minority opinion spreading
We study the effect of finite size population in Galam's model [Eur. Phys. J.
B 25 (2002) 403] of minority opinion spreading and introduce neighborhood
models that account for local spatial effects. For systems of different sizes
N, the time to reach consensus is shown to scale as ln N in the original
version, while the evolution is much slower in the new neighborhood models. The
threshold value of the initial concentration of minority supporters for the
defeat of the initial majority, which is independent of N in Galam's model,
goes to zero with growing system size in the neighborhood models. This is a
consequence of the existence of a critical size for the growth of a local
domain of minority supporters
- …