982 research outputs found
Einstein static universes are unstable in generic f(R) models
We study Einstein static universes in the context of generic f(R) models. It
is shown that Einstein static solutions exist for a wide variety of modified
gravity models sourced by a barotropic perfect fluid with equation of state
w=p/rho, but these solutions are always unstable to either homogeneous or
inhomogeneous perturbations. Our general results are in agreement with specific
models investigated in that past. We also discuss how our techniques can be
applied to other scenarios in f(R) gravity.Comment: 7 pages, 2 figures. Minor corrections. Minor changes and references
added to match version accepted by Phys. Rev.
Extra force in modified theories of gravity
The equation of motion for test particles in modified theories of
gravity is derived. By considering an explicit coupling between an arbitrary
function of the scalar curvature, , and the Lagrangian density of matter, it
is shown that an extra force arises. This extra force is orthogonal to the
four-velocity and the corresponding acceleration law is obtained in the weak
field limit. Connections with MOND and with the Pioneer anomaly are further
discussed.Comment: Revtex4 file, 5 pages. Version to appear in Physical Review
Origin of the tetragonal-to-orthorhombic (nematic) phase transition in FeSe: a combined thermodynamic and NMR study
The nature of the tetragonal-to-orthorhombic structural transition at
K in single crystalline FeSe is studied using shear-modulus,
heat-capacity, magnetization and NMR measurements. The transition is shown to
be accompanied by a large shear-modulus softening, which is practically
identical to that of underdoped Ba(Fe,Co)As, suggesting very similar
strength of the electron-lattice coupling. On the other hand, a
spin-fluctuation contribution to the spin-lattice relaxation rate is only
observed below . This indicates that the structural, or "nematic", phase
transition in FeSe is not driven by magnetic fluctuations
Galactic cold dark matter as a Bose-Einstein condensate of WISPs
We propose here the dark matter content of galaxies as a cold bosonic fluid
composed of Weakly Interacting Slim Particles (WISPs), represented by spin-0
axion-like particles and spin-1 hidden bosons, thermalized in the Bose-Einstein
condensation state and bounded by their self-gravitational potential. We
analyze two zero-momentum configurations: the polar phases in which spin
alignment of two neighbouring particles is anti-parallel and the ferromagnetic
phases in which every particle spin is aligned in the same direction. Using the
mean field approximation we derive the Gross-Pitaevskii equations for both
cases, and, supposing the dark matter to be a polytropic fluid, we describe the
particles density profile as Thomas-Fermi distributions characterized by the
halo radii and in terms of the scattering lengths and mass of each particle. By
comparing this model with data obtained from 42 spiral galaxies and 19 Low
Surface Brightness (LSB) galaxies, we constrain the dark matter particle mass
to the range and we find the lower bound for the
scattering length to be of the order .Comment: 13 pages; 6 figures; references added; v.3: typo corrected in the
abstract, published in JCA
A new pseudopolymorph of 5,11,17,23,29,35,41,47-octabromo-49,50,51,52,53, 54,55,56-octamethoxycalix[8]arene
Crystals of the title calixarene containing CCl4 as solvent have already been investigated [Baudry, Felix, Bavoux, Perrin, Vocanson, Dumazet-Bonnamour & Lamartine (2003). New J. Chem. pp. 1540-1543]. We present here a new pseudopolymorph of this compound, C64H 56Br8O8·2C4H8O, containing tetrahydrofuran as solvent and crystallizing in a different space group. However, the molecular conformation of both molecules is very similar. Further similarities of both structures are that the calixarene molecules are located on a centre of inversion and that the solvent molecules occupy similar positions with respect to the calixarene molecules. © 2003 International Union of Crystallography Printed in Great Britain - all rights reserved
Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by 75As NMR
We report As nuclear magnetic resonance (NMR) studies on a new
iron-based superconductor CaKFeAs with = 35 K. As
NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites
close to the K and Ca layers, respectively, revealing that K and Ca layers are
well ordered without site inversions. We found that nuclear quadrupole
frequencies of the As(1) and As(2) sites show an opposite
temperature () dependence. Nearly independent behavior of the Knight
shifts are observed in the normal state, and a sudden decrease in in
the superconducting (SC) state clearly evidences spin-singlet Cooper pairs.
As spin-lattice relaxation rates 1/ show a power law dependence
with different exponents for the two As sites. The isotropic antiferromagnetic
spin fluctuations characterized by the wavevector = (, 0) or (0,
) in the single-iron Brillouin zone notation are revealed by 1/ and
measurements. Such magnetic fluctuations are necessary to explain the
observed temperature dependence of the As quadrupole frequencies, as
evidenced by our first-principles calculations. In the SC state, 1/ shows
a rapid decrease below without a Hebel-Slichter peak and decreases
exponentially at low , consistent with an nodeless two-gap
superconductor.Comment: 9 pages, 6 figures, accepted for publication in Phys.Rev.
Dark energy with dark spinors
Ever since the first observations that we are living in an accelerating
universe, it has been asked what dark energy is. There are various explanations
all of which with have various draw backs or inconsistencies. Here we show that
using a dark spinor field it is possible to have an equation of state that
crosses the phantom divide, becoming a dark phantom spinor which evolves into
dark energy. This type of equation of state has been mildly favored by
experimental data, however, in the past there were hardly any candidate
theories that satisfied this crossing without creating ghosts or causing a
singularity which results in the universe essentially ripping. The dark spinor
model converges to dark energy in a reasonable time frame avoiding the big rip
and without attaining negative kinetic energy as it crosses the phantom divide.Comment: 12 pages, 7 figure
Reply to ``Comment on `Hole-burning experiments within glassy models with infinite range interactions' ''
This is a reply to the comments by Richter and Chamberlin, and Diezemann and
Bohmer to our paper (Phys. Rev. Lett. 85, 3448 (2000)). As further evidence for
the claims in this Letter, we here reproduce the nonlinear spectral
hole-burning experimental protocol in an equilibrated fully connected
spin-glass model and we exhibit frequency selectivity, together with a shift in
the base of the spectral hole.Comment: 1 page, two figures, to appear in Phys. Rev. Let
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