13,548 research outputs found
Dynamic fluctuations in unfrustrated systems: random walks, scalar fields and the Kosterlitz-Thouless phase
We study analytically the distribution of fluctuations of the quantities
whose average yield the usual two-point correlation and linear response
functions in three unfrustrated models: the random walk, the dimensional
scalar field and the 2d XY model. In particular we consider the time dependence
of ratios between composite operators formed with these fluctuating quantities
which generalize the largely studied fluctuation-dissipation ratio, allowing us
to discuss the relevance of the effective temperature notion beyond linear
order. The behavior of fluctuations in the aforementioned solvable cases is
compared to numerical simulations of the 2d clock model with states.Comment: 27 pages, 3 figure
Out of equilibrium dynamics of the spiral model
We study the relaxation of the bi-dimensional kinetically constrained spiral
model. We show that due to the reversibility of the dynamic rules any unblocked
state fully decorrelates in finite times irrespectively of the system being in
the unjammed or the jammed phase. In consequence, the evolution of any
unblocked configuration occurs in a different sector of phase space from the
one that includes the equilibrium blocked equilibrium configurations at
criticality and in the jammed phase. We argue that such out of equilibrium
dynamics share many points in common with coarsening in the one-dimensional
Ising model and we identify the coarsening structures that are, basically,
lines of vacancies. We provide evidence for this claim by analyzing the
behaviour of several observables including the density of particles and
vacancies, the spatial correlation function, the time-dependent persistence and
the linear response.Comment: 14 pages 12 figure
An internal mechanism for the anti-glitch observed in AXP 1E 2259+586
Magnetars are fascinating objects that are thought to be neutron stars
powered by their strong internal magnetic fields. Clear evidence of a sudden
spin-down was detected in the Anomalous X-ray Pulsar AXP 1E 2259+586, an object
cataloged as a magnetar. This event received the name "anti-glitch". In this
work we present a simple internal mechanism which could account for the
observed sudden spin-down of the star.Comment: Presented in the "Compact Stars in the QCD Phase Diagram IV"
conference. September 26-30, 2014, Prerow, German
Variational wave functions for the Heisenberg model on the anisotropic triangular lattice: Spin liquids and spiral orders
By using variational wave functions and quantum Monte Carlo techniques, we
investigate the complete phase diagram of the Heisenberg model on the
anisotropic triangular lattice, where two out of three bonds have
super-exchange couplings and the third one has instead . This
model interpolates between the square lattice and the isotropic triangular one,
for , and between the isotropic triangular lattice and a set
of decoupled chains, for . We consider all the
fully-symmetric spin liquids that can be constructed with the fermionic
projective-symmetry group classification [Y. Zhou and X.-G. Wen,
arXiv:cond-mat/0210662] and we compare them with the spiral magnetic orders
that can be accommodated on finite clusters. Our results show that, for
, the phase diagram is dominated by magnetic orderings, even
though a spin-liquid state may be possible in a small parameter window, i.e.,
. In contrast, for , a
large spin-liquid region appears close to the limit of decoupled chains, i.e.,
for , while magnetically ordered phases with spiral
order are stabilized close to the isotropic point.Comment: 11 pages, 11 figure
Acquisition of ownership illusion with self-disownership in neurological patients
The multisensory regions in frontoparietal cortices play a crucial role in the sense of body and self. Disrupting this sense may lead to a feeling of disembodiment, or more generally, a sense of disownership. Experimentally, this altered consciousness disappears during illusory own-body perceptions, increasing the intensity of perceived ownership for an external virtual limb. In many clinical conditions, particularly in individuals with a discontinuous or absent sense of bodily awareness, the brain may effortlessly create a convincing feeling of body ownership over a surrogate body or body part. The immediate visual input dominates the current bodily state and induces rapid plastic adaptation that reconfigures the dynamics of bodily representation, allowing the brain to acquire an alternative sense of body and self. Investigating strategies to deconstruct the lack of a normal sense of bodily ownership, especially after a neurological injury, may aid the selection of appropriate clinical treatment
Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots
Context. Quasi-periodic variability has been observed in a number of X-ray
binaries harboring black hole candidates. In general relativity, black holes
are uniquely described by the Kerr metric and, according to the cosmic
censorship conjecture, curvature singularities always have to be clothed by an
event horizon. Aims. In this paper, we study the effect of an external magnetic
field on the observed light curves of orbiting hot spots in thin accretion
discs around Kerr black holes and naked singularities. Methods. We employ a
ray-tracing algorithm to calculate the light curves and power spectra of such
hot spots as seen by a distant observer for uniform and dipolar magnetic field
configurations assuming a weak coupling between the magnetic field and the disc
matter. Results. We show that the presence of an external dipolar magnetic
field leads to potentially observable modifications of these signals for both
Kerr black holes and naked singularities, while an external uniform magnetic
field has practically no effect. In particular, we demonstrate that the
emission from a hot spot orbiting near the innermost stable circular orbit of a
naked singularity in a dipolar magnetic field can be significantly harder than
the emission of the same hot spot in the absence of such a magnetic field.
Conclusions. The comparison of our model with observational data may allow us
study the geometry of magnetic fields around compact objects and to test the
cosmic censorship conjecture in conjunction with other observables such as
thermal continuum spectra and iron line profiles.Comment: 7 pages, 3 figures, submitted to Astronomy and Astrophysic
Local Observables in a Landscape of Infrared Gauge Modes
Cosmological local observables are at best statistically determined by the
fundamental theory describing inflation. When the scalar inflaton is coupled
uniformly to a collection of subdominant massless gauge vectors, rotational
invariance is obeyed locally. However, the statistical isotropy of fluctuations
is spontaneously broken by gauge modes whose wavelength exceed our causal
horizon. This leads to a landscape picture where primordial correlators depend
on the position of the observer. We compute the stochastic corrections to the
curvature power spectrum, show the existence of a new local observable (the
shape of the quadrupole), and constrain the theory using Planck limits.Comment: 5 pages, 3 figures, v2: minor updates, matches version published in
Physics Letters
Numerical integration of the stochastic Landau-Lifshitz-Gilbert equation in generic time-discretisation schemes
We introduce a numerical method to integrate the stochastic
Landau-Lifshitz-Gilbert equation in spherical coordinates for generic
discretization schemes. This method conserves the magnetization modulus and
ensures the approach to equilibrium under the expected conditions. We test the
algorithm on a benchmark problem: the dynamics of a uniformly magnetized
ellipsoid. We investigate the influence of various parameters, and in
particular, we analyze the efficiency of the numerical integration, in terms of
the number of steps needed to reach a chosen long time with a given accuracy.Comment: 9 pages and 7 figure
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