220 research outputs found
The path integral measure, constraints and ghosts for massive gravitons with a cosmological constant
For massive gravity in a de Sitter background one encounters problems of
stability when the curvature is larger than the graviton mass. I analyze this
situation from the path integral point of view and show that it is related to
the conformal factor problem of Euclidean quantum (massless) gravity. When a
constraint for massive gravity is incorporated and the proper treatment of the
path integral measure is taken into account one finds that, for particular
choices of the DeWitt metric on the space of metrics (in fact, the same choices
as in the massless case), one obtains the opposite bound on the graviton mass.Comment: LaTeX, 10 pages, to appear in Phys. Rev.
Hard thermal loops with a background plasma velocity
I consider the calculation of the two and three-point functions for QED at
finite temperature in the presence of a background plasma velocity. The final
expressions are consistent with Lorentz invariance, gauge invariance and
current conservation, pointing to a straightforward generalization of the hard
thermal loop formalism to this physical situation. I also give the resulting
expression for the effective action and identify the various terms.Comment: 11 pages, no figure
Plasmon interactions in the quark-gluon plasma
Yang-Mills theory at finite temperature is rewritten as a theory of plasmons
which provides a Hamiltonian framework for perturbation theory with resummation
of hard thermal loops.Comment: 12 pages, LaTeX, minor typos corrected, discussion adde
Dynamic binding of driven interfaces in coupled ultrathin ferromagnetic layers
We demonstrate experimentally dynamic interface binding in a system
consisting of two coupled ferromagnetic layers. While domain walls in each
layer have different velocity-field responses, for two broad ranges of the
driving field, H, walls in the two layers are bound and move at a common
velocity. The bound states have their own velocity-field response and arise
when the isolated wall velocities in each layer are close, a condition which
always occurs as H->0. Several features of the bound states are reproduced
using a one dimensional model, illustrating their general nature.Comment: 5 pages, 4 figures, to be published in Physical Review Letter
Highly asymmetric magnetic domain wall propagation due to coupling to a periodic pinning potential
Magneto-optical microscopy and magnetometry have been used to study
19 magnetization reversal in an ultrathin magnetically soft [Pt/Co]2 ferromagnetic film
20 coupled to an array of magnetically harder [Co/Pt]4 nanodots via a predominantly
21 dipolar interaction across a 3 nm Pt spacer. This interaction generates a spatially
22 periodic pinning potential for domain walls propagating through the continuous
23 magnetic film. When reversing the applied field with respect to the static nanodot
24 array magnetization orientation, strong asymmetries in the wall velocity and switching
25 fields are observed. Asymmetric switching fields mean that the hysteresis of the film is
26 characterized by a large bias field of dipolar origin which is linked to the wall velocity
27 asymmetry. This latter asymmetry, though large at low fields, vanishes at high fields
28 where the domains become round and compact. A field-polarity-controlled transition
29 from dendritic to compact faceted domain structures is also seen at low field and a
30 model is proposed to interpret the transition
Aging dynamics of non-linear elastic interfaces: the Kardar-Parisi-Zhang equation
In this work, the out-of-equilibrium dynamics of the Kardar-Parisi-Zhang
equation in (1+1) dimensions is studied by means of numerical simulations,
focussing on the two-times evolution of an interface in the absence of any
disordered environment. This work shows that even in this simple case, a rich
aging behavior develops. A multiplicative aging scenario for the two-times
roughness of the system is observed, characterized by the same growth exponent
as in the stationary regime. The analysis permits the identification of the
relevant growing correlation length, accounting for the important scaling
variables in the system. The distribution function of the two-times roughness
is also computed and described in terms of a generalized scaling relation.
These results give good insight into the glassy dynamics of the important case
of a non-linear elastic line in a disordered medium.Comment: 14 pages, 6 figure
Spatially periodic domain wall pinning potentials: Asymmetric pinning and dipolar biasing
Domain wall propagation has been measured in continuous, weakly disordered,
quasi-two-dimensional, Ising-like magnetic layers that are subject to spatially
periodic domain wall pinning potentials. The potentials are generated
non-destructively using the stray magnetic field of ordered arrays of
magnetically hard [Co/Pt] nanoplatelets which are patterned above and are
physically separated from the continuous magnetic layer. The effect of the
periodic pinning potentials on thermally activated domain wall creep dynamics
is shown to be equivalent, at first approximation, to that of a uniform,
effective retardation field, , which acts against the applied field,
. We show that depends not only on the array geometry but also on
the relative orientation of and the magnetization of the nanoplatelets. A
result of the latter dependence is that wall-mediated hysteresis loops obtained
for a set nanoplatelet magnetization exhibit many properties that are normally
associated with ferromagnet/antiferromagnet exchange bias systems. These
include a switchable bias, coercivity enhancement and domain wall roughness
that is dependent on the applied field polarity.Comment: 12 pages, 9 figure
A meta-analysis of state-of-the-art electoral prediction from Twitter data
Electoral prediction from Twitter data is an appealing research topic. It
seems relatively straightforward and the prevailing view is overly optimistic.
This is problematic because while simple approaches are assumed to be good
enough, core problems are not addressed. Thus, this paper aims to (1) provide a
balanced and critical review of the state of the art; (2) cast light on the
presume predictive power of Twitter data; and (3) depict a roadmap to push
forward the field. Hence, a scheme to characterize Twitter prediction methods
is proposed. It covers every aspect from data collection to performance
evaluation, through data processing and vote inference. Using that scheme,
prior research is analyzed and organized to explain the main approaches taken
up to date but also their weaknesses. This is the first meta-analysis of the
whole body of research regarding electoral prediction from Twitter data. It
reveals that its presumed predictive power regarding electoral prediction has
been rather exaggerated: although social media may provide a glimpse on
electoral outcomes current research does not provide strong evidence to support
it can replace traditional polls. Finally, future lines of research along with
a set of requirements they must fulfill are provided.Comment: 19 pages, 3 table
Spin density wave dislocation in chromium probed by coherent x-ray diffraction
We report on the study of a magnetic dislocation in pure chromium. Coherent
x-ray diffraction profiles obtained on the incommensurate Spin Density Wave
(SDW) reflection are consistent with the presence of a dislocation of the
magnetic order, embedded at a few micrometers from the surface of the sample.
Beyond the specific case of magnetic dislocations in chromium, this work may
open up a new method for the study of magnetic defects embedded in the bulk.Comment: 8 pages, 7 figure
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