4,769 research outputs found
Higher Order Effects in the Dielectric Constant of Percolative Metal-Insulator Systems above the Critical Point
The dielectric constant of a conductor-insulator mixture shows a pronounced
maximum above the critical volume concentration. Further experimental evidence
is presented as well as a theoretical consideration based on a phenomenological
equation. Explicit expressions are given for the position of the maximum in
terms of scaling parameters and the (complex) conductances of the conductor and
insulator. In order to fit some of the data, a volume fraction dependent
expression for the conductivity of the more highly conductive component is
introduced.Comment: 4 pages, Latex, 4 postscript (*.epsi) files submitted to Phys Rev.
One-loop effective potential in M4 x T2 with and without 't Hooft flux
We review the basic notions of compactification in the presence of a
background flux. In extra-dimentional models with more than five dimensions,
Scherk and Schwarz boundary conditions have to satisfy 't Hooft consistency
conditions. Different vacuum configurations can be obtained, depending whether
trivial or non-trivial 't Hooft flux is considered. The presence of the
"magnetic" background flux provide, in addition, a mechanism for producing
four-dimensional chiral fermions. Particularizing to the six-dimensional case,
we calculate the one-loop effective potential for a U(N) gauge theory on M4 x
T2. We firstly review the well known results of the trivial 't Hooft flux case,
where one-loop contributions produce the usual Hosotani dynamical symmetry
breaking. Finally we applied our result for describing, for the first time, the
one-loop contributions in the non-trivial 't Hooft flux case
Nonequilibrium Atom-Dielectric Forces Mediated by a Quantum Field
In this paper we give a first principles microphysics derivation of the
nonequilibrium forces between an atom, treated as a three dimensional harmonic
oscillator, and a bulk dielectric medium modeled as a continuous lattice of
oscillators coupled to a reservoir. We assume no direct interaction between the
atom and the medium but there exist mutual influences transmitted via a common
electromagnetic field. By employing concepts and techniques of open quantum
systems we introduce coarse-graining to the physical variables - the medium,
the quantum field and the atom's internal degrees of freedom, in that order -
to extract their averaged effects from the lowest tier progressively to the top
tier. The first tier of coarse-graining provides the averaged effect of the
medium upon the field, quantified by a complex permittivity (in the frequency
domain) describing the response of the dielectric to the field in addition to
its back action on the field through a stochastic forcing term. The last tier
of coarse- graining over the atom's internal degrees of freedom results in an
equation of motion for the atom's center of mass from which we can derive the
force on the atom. Our nonequilibrium formulation provides a fully dynamical
description of the atom's motion including back action effects from all other
relevant variables concerned. In the long-time limit we recover the known
results for the atom-dielectric force when the combined system is in
equilibrium or in a nonequilibrium stationary state.Comment: 24 pages, 2 figure
Matter-screened Casimir force and Casimir-Polder force in planar structures
Using a recently developed theory of the Casimir force (Raabe C and Welsch
D-G 2005 Phys. Rev. A 71 013814), we calculate the force that acts on a plate
in front of a planar wall and the force that acts on the plate in the case
where the plate is part of matter that fills the space in front of the wall. We
show that in the limit of a dielectric plate whose permittivity is close to
unity, the force obtained in the former case reduces to the ordinary, i.e.,
unscreened Casimir-Polder force acting on isolated atoms. In the latter case,
the theory yields the Casimir-Polder force that is screened by the surrounding
matter.Comment: 11 pages, 1 figure -- published online at J. Opt. B on Nov 16 200
Enhanced dispersion interaction in confined geometry
The dispersion interaction between two point-like particles confined in a
dielectric slab between two plates of another dielectric medium is studied
within a continuum (Lifshitz) theory. The retarded (Casimir-Polder) interaction
at large inter-particle distances is found to be strongly enhanced as the
mismatch between the dielectric permittivities of the two media is increased.
The large-distance interaction is multiplied due to confinement by a factor of
at zero temperature, and by
at finite temperature, \gamma=\ein(0)/\eout(0)
being the ratio between the static dielectric permittivities of the inner and
outer media. This confinement-induced amplification of the dispersion
interaction can reach several orders of magnitude.Comment: 4 page
Can Euroscepticism Contribute to a European Public Sphere? The Europeanization of Media Discourses about Euroscepticism across Six Countries
This study compares the media discourses about Euroscepticism in 2014 across
six countries (United Kingdom, Ireland, France, Spain, Sweden, and Denmark). We
assess the extent to which the mass media's reporting of Euroscepticism
indicates the Europeanization of public spheres. Using a mixed-methods approach
combining LDA topic modeling and qualitative coding, we find that approximately
70 per cent of print articles mentioning "Euroscepticism" or "Eurosceptic" are
framed in a non-domestic (i.e. European) context. In five of the six cases
studied, articles exhibiting a European context are strikingly similar in
content, with the British case as the exception. However, coverage of British
Euroscepticism drives Europeanization in other Member States. Bivariate
logistic regressions further reveal three macro-level structural variables that
significantly correlate with a Europeanized media discourse: newspaper type
(tabloid or broadsheet), presence of a strong Eurosceptic party, and
relationship to the EU budget (net contributor or receiver of EU funds).Comment: 29 pages, 2 figures, 4 tables, 2 appendice
Quantum corrections from a path integral over reparametrizations
We study the path integral over reparametrizations that has been proposed as
an ansatz for the Wilson loops in the large- QCD and reproduces the area law
in the classical limit of large loops. We show that a semiclassical expansion
for a rectangular loop captures the L\"uscher term associated with
dimensions and propose a modification of the ansatz which reproduces the
L\"uscher term in other dimensions, which is observed in lattice QCD. We repeat
the calculation for an outstretched ellipse advocating the emergence of an
analog of the L\"uscher term and verify this result by a direct computation of
the determinant of the Laplace operator and the conformal anomaly
Inferring the Origin Locations of Tweets with Quantitative Confidence
Social Internet content plays an increasingly critical role in many domains,
including public health, disaster management, and politics. However, its
utility is limited by missing geographic information; for example, fewer than
1.6% of Twitter messages (tweets) contain a geotag. We propose a scalable,
content-based approach to estimate the location of tweets using a novel yet
simple variant of gaussian mixture models. Further, because real-world
applications depend on quantified uncertainty for such estimates, we propose
novel metrics of accuracy, precision, and calibration, and we evaluate our
approach accordingly. Experiments on 13 million global, comprehensively
multi-lingual tweets show that our approach yields reliable, well-calibrated
results competitive with previous computationally intensive methods. We also
show that a relatively small number of training data are required for good
estimates (roughly 30,000 tweets) and models are quite time-invariant
(effective on tweets many weeks newer than the training set). Finally, we show
that toponyms and languages with small geographic footprint provide the most
useful location signals.Comment: 14 pages, 6 figures. Version 2: Move mathematics to appendix, 2 new
references, various other presentation improvements. Version 3: Various
presentation improvements, accepted at ACM CSCW 201
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