1,458 research outputs found
Perfect fluid spheres with cosmological constant
We examine static perfect fluid spheres in the presence of a cosmological
constant. New exact matter solutions are discussed which require the Nariai
metric in the vacuum region. We generalize the Einstein static universe such
that neither its energy density nor its pressure is constant throughout the
spacetime. Using analytical techniques we derive conditions depending on the
equation of state to locate the vanishing pressure surface. This surface can in
general be located in regions with decreasing area group orbits. We use
numerical methods to integrate the field equations for realistic equations of
state and find consistent results.Comment: 15 pages, 6 figures; added new references, removed one figure,
improved text, accepted for publication in PR
Bounds on the basic physical parameters for anisotropic compact general relativistic objects
We derive upper and lower limits for the basic physical parameters
(mass-radius ratio, anisotropy, redshift and total energy) for arbitrary
anisotropic general relativistic matter distributions in the presence of a
cosmological constant. The values of these quantities are strongly dependent on
the value of the anisotropy parameter (the difference between the tangential
and radial pressure) at the surface of the star. In the presence of the
cosmological constant, a minimum mass configuration with given anisotropy does
exist. Anisotropic compact stellar type objects can be much more compact than
the isotropic ones, and their radii may be close to their corresponding
Schwarzschild radii. Upper bounds for the anisotropy parameter are also
obtained from the analysis of the curvature invariants. General restrictions
for the redshift and the total energy (including the gravitational
contribution) for anisotropic stars are obtained in terms of the anisotropy
parameter. Values of the surface redshift parameter greater than two could be
the main observational signature for anisotropic stellar type objects.Comment: 18 pages, no figures, accepted for publication in CQ
A new two-sphere singularity in general relativity
The Florides solution, proposed as an alternative to the interior
Schwarzschild solution, represents a static and spherically symmetric geometry
with vanishing radial stresses. It is regular at the center, and is matched to
an exterior Schwarzschild solution. The specific case of a constant energy
density has been interpreted as the field inside an Einstein cluster. In this
work, we are interested in analyzing the geometry throughout the permitted
range of the radial coordinate without matching it to the Schwarzschild
exterior spacetime at some constant radius hypersurface. We find an interesting
picture, namely, the solution represents a three-sphere, whose equatorial
two-sphere is singular, in the sense that the curvature invariants and the
tangential pressure diverge. As far as we know, such singularities have not
been discussed before. In the presence of a large negative cosmological
constant (anti-de Sitter) the singularity is removed.Comment: 17 pages, 3 figure
Nonlinear response theory for Markov processes: Simple models for glassy relaxation
The theory of nonlinear response for Markov processes obeying a master
equation is formulated in terms of time-dependent perturbation theory for the
Green's functions and general expressions for the response functions up to
third order in the external field are given. The nonlinear response is
calculated for a model of dipole reorientations in an asymmetric double well
potential, a standard model in the field of dielectric spectroscopy. The static
nonlinear response is finite with the exception of a certain temperature
determined by the value of the asymmetry. In a narrow temperature range around
, the modulus of the frequency-dependent cubic response shows a peak at a
frequency on the order of the relaxation rate and it vanishes for both, low
frequencies and high frequencies. At temperatures at which the static response
is finite (lower and higher than ), the modulus is found to decay
monotonously from the static limit to zero at high frequencies. In addition,
results of calculations for a trap model with a Gaussian density of states are
presented. In this case, the cubic response depends on the specific dynamical
variable considered and also on the way the external field is coupled to the
kinetics of the model. In particular, a set of different dynamical variables is
considered that gives rise to identical shapes of the linear susceptibility and
only to different temperature dependencies of the relaxation times. It is found
that the frequency dependence of the nonlinear response functions, however,
strongly depends on the particular choice of the variables. The results are
discussed in the context of recent theoretical and experimental findings
regarding the nonlinear response of supercooled liquids and glasses.Comment: 23 pages, 10 figure
Response-theory for nonresonant hole burning: Stochastic dynamics
Using non-linear response theory the time signals relevant for nonresonant
spectral hole burning are calculated. The step-reponse function following the
application of a high amplitude ac field (pump) and an intermediate waiting
period is shown to be the sum of the equilibrium integrated response and a
modification due to the preparation via ac irradiation. Both components are
calculated for a class of stochastic dipole reorientation models. The results
indicate that the method can be used for a clearcut distinction of
homogeneously and heterogeneously broadened susceptibilities as they occur in
the relaxation of supercooled liquids or other disordered materials. This is
because only in the heterogeneous case is a frequency selective modification of
the response possible.Comment: revised version, 7 pages, 2 figure
Origin of non-exponential relaxation in a crystalline ionic conductor: a multi-dimensional 109Ag NMR study
The origin of the non-exponential relaxation of silver ions in the
crystalline ion conductor Ag7P3S11 is analyzed by comparing appropriate
two-time and three-time 109Ag NMR correlation functions. The non-exponentiality
is due to a rate distribution, i.e., dynamic heterogeneities, rather than to an
intrinsic non-exponentiality. Thus, the data give no evidence for the relevance
of correlated back-and-forth jumps on the timescale of the silver relaxation.Comment: 4 pages, 3 figure
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
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