2,056 research outputs found
Critical behavior of dissipative two-dimensional spin lattices
We explore critical properties of two-dimensional lattices of spins
interacting via an anisotropic Heisenberg Hamiltonian and subject to incoherent
spin flips. We determine the steady-state solution of the master equation for
the density matrix via the corner-space renormalization method. We investigate
the finite-size scaling and critical exponent of the magnetic linear
susceptibility associated to a dissipative ferromagnetic transition. We show
that the Von Neumann entropy increases across the critical point, revealing a
strongly mixed character of the ferromagnetic phase. Entanglement is witnessed
by the quantum Fisher information which exhibits a critical behavior at the
transition point, showing that quantum correlations play a crucial role in the
transition even though the system is in a mixed state.Comment: Accepted for publication on Phys. Rev. B (6 pages, 5 figures
The Maximal Amount of Gravitational Waves in the Curvaton Scenario
The curvaton scenario for the generation of the cosmological curvature
perturbation on large scales represents an alternative to the standard
slow-roll scenario of inflation in which the observed density perturbations are
due to fluctuations of the inflaton field itself. Its basic assumption is that
the initial curvature perturbation due to the inflaton field is negligible.
This is attained by lowering the energy scale of inflation, thereby highly
suppressing the amount of gravitational waves produced during inflation. We
compute the power-spectrum of the gravitational waves generated at second order
in perturbation theory by the curvaton (isocurvature) perturbations between the
end of inflation and the curvaton decay. An interesting property of this
contribution to the tensor perturbations is that it is directly proportional to
the amount of non-Gaussianity predicted within the curvaton scenario. We show
that the spectrum of gravitational waves may be in the range of future
gravitational wave detectors.Comment: 4 pages, laTeX; added a clarifying comment in the conclusions,
version matches publication in PRD, Rapid Communication
CMB Anisotropies at Second Order I
We present the computation of the full system of Boltzmann equations at
second-order describing the evolution of the photon, baryon and cold dark
matter fluids. These equations allow to follow the time evolution of the Cosmic
Microwave Background (CMB) anisotropies at second-order at all angular scales
from the early epoch, when the cosmological perturbations were generated, to
the present through the recombination era. This paper sets the stage for the
computation of the full second-order radiation transfer function at all scales
and for a a generic set of initial conditions specifying the level of
primordial non-Gaussianity. In a companion paper, we will present the
computation of the three-point correlation function at recombination which is
so relevant for the issue of non-Gaussianity in the CMB anisotropies.Comment: 26 pages, LaTeX file, typos correcte
On the non-Gaussianity from Recombination
The non-linear effects operating at the recombination epoch generate a
non-Gaussian signal in the CMB anisotropies. Such a contribution is relevant
because it represents a major part of the second-order radiation transfer
function which must be determined in order to have a complete control of both
the primordial and non-primordial part of non-Gaussianity in the CMB
anisotropies. We provide an estimate of the level of non-Gaussianity in the CMB
arising from the recombination epoch which shows up mainly in the equilateral
configuration. We find that it causes a contamination to the possible
measurement of the equilateral primordial bispectrum shifting the minimum
detectable value of the non-Gaussian parameter f^equil_NL by Delta f^equil_NL=
O(10) for an experiment like Planck.Comment: LaTeX file; 11 pages. v2: Typos corrected; references added; comments
about the effective non-linearity parameter added in Sec. IV; comments added
in the conclusions of Sec. IV. v3: References added; some clarifications
added as footnotes 4 and 6, and in Sec. 3. Matches version accepted for
publication in JCA
Non-Gaussianity in the Cosmic Microwave Background Anisotropies at Recombination in the Squeezed limit
We estimate analytically the second-order cosmic microwave background
temperature anisotropies at the recombination epoch in the squeezed limit and
we deduce the contamination to the primordial local non-Gaussianity. We find
that the level of contamination corresponds to f_NL^{con}=O(1) which is below
the sensitivity of present experiments and smaller than the value O(5) recently
claimed in the literature.Comment: LaTeX file; 15 pages. Slightly revised version. Main result unchange
High order correlation functions for self interacting scalar field in de Sitter space
We present the expressions of the three- and four-point correlation functions
of a self interacting light scalar field in a de Sitter spacetime at tree order
respectively for a cubic and a quartic potential. Exact expressions are derived
and their limiting behaviour on super-horizon scales are presented. Their
essential features are shown to be similar to those obtained in a classical
approach.Comment: 8 pages, 4 figure
Infinitesimal and local convexity of a hypersurface in a semi-Riemannian manifold
Given a Riemannian manifold M and a hypersurface H in M, it is well known
that infinitesimal convexity on a neighborhood of a point in H implies local
convexity. We show in this note that the same result holds in a semi-Riemannian
manifold. We make some remarks for the case when only timelike, null or
spacelike geodesics are involved. The notion of geometric convexity is also
reviewed and some applications to geodesic connectedness of an open subset of a
Lorentzian manifold are given.Comment: 14 pages, AMSLaTex, 2 figures. v2: typos fixed, added one reference
and several comments, statement of last proposition correcte
Bouncing or sticky droplets: impalement transitions on superhydrophobic micropatterned surfaces
When a liquid drops impinges a hydrophobic rough surface it can either bounce
off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel
droplets). The analysis of drop impact and quasi static ''loading'' experiments
on model microfabricated surfaces allows to clearly identify the forces
hindering the impalement transitions. A simple semi-quantitative model is
proposed to account for the observed relation between the surface topography
and the robustness of fakir non-wetting states. Motivated by potential
applications in microfluidics and in the fabrication of self cleaning surfaces,
we finally propose some guidelines to design robust superhydrophobic surfaces.Comment: 7 pages, 5 figure
A new approach to cosmological perturbations in f(R) models
We propose an analytic procedure that allows to determine quantitatively the
deviation in the behavior of cosmological perturbations between a given f(R)
modified gravity model and a LCDM reference model. Our method allows to study
structure formation in these models from the largest scales, of the order of
the Hubble horizon, down to scales deeply inside the Hubble radius, without
employing the so-called "quasi-static" approximation. Although we restrict our
analysis here to linear perturbations, our technique is completely general and
can be extended to any perturbative order.Comment: 21 pages, 2 figures; Revised version according to reviewer's
suggestions; Typos corrected; Added Reference
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