1,118 research outputs found
Gravitational Properties of Monopole Spacetimes Near the Black Hole Threshold
Although nonsingular spacetimes and those containing black holes are
qualitatively quite different, there are continuous families of configurations
that connect the two. In this paper we use self-gravitating monopole solutions
as tools for investigating the transition between these two types of
spacetimes. We show how causally distinct regions emerge as the black hole
limit is achieved, even though the measurements made by an external observer
vary continuously. We find that near-critical solutions have a naturally
defined entropy, despite the absence of a true horizon, and that this has a
clear connection with the Hawking-Bekenstein entropy. We find that certain
classes of near-critical solutions display naked black hole behavior, although
they are not truly black holes at all. Finally, we present a numerical
simulation illustrating how an incident pulse of matter can induce the
dynamical collapse of a monopole into an extremal black hole. We discuss the
implications of this process for the third law of black hole thermodynamics.Comment: 23 pages, 4 figures RevTe
Scaling of Star Polymers with one to 80 Arms
We present large statistics simulations of 3-dimensional star polymers with
up to arms, and with up to 4000 monomers per arm for small values of
. They were done for the Domb-Joyce model on the simple cubic lattice. This
is a model with soft core exclusion which allows multiple occupancy of sites
but punishes each same-site pair of monomers with a Boltzmann factor . We
use this to allow all arms to be attached at the central site, and we use the
`magic' value to minimize corrections to scaling. The simulations are
made with a very efficient chain growth algorithm with resampling, PERM,
modified to allow simultaneous growth of all arms. This allows us to measure
not only the swelling (as observed from the center-to-end distances), but also
the partition sum. The latter gives very precise estimates of the critical
exponents . For completeness we made also extensive simulations of
linear (unbranched) polymers which give the best estimates for the exponent
.Comment: 7 pages, 7 figure
Weak gravity in DGP braneworld model
We analyze the weak gravity in the braneworld model proposed by
Dvali-Gabadadze-Porrati, in which the unperturbed background spacetime is given
by five dimensional Minkowski bulk with a brane which has the induced Einstein
Hilbert term. This model has a critical length scale . Naively, we expect
that the four dimensional general relativity (4D GR) is approximately recovered
at the scale below . However, the simple linear perturbation does not work
in this regime. Only recently the mechanism to recover 4D GR was clarified
under the restriction to spherically symmetric configurations, and the leading
correction to 4D GR was derived. Here, we develop an alternative formulation
which can handle more general perturbations. We also generalize the model by
adding bulk cosmological constant and the brane tension.Comment: 7 pages, 1 figure, references adde
Star tracks in the ghost condensate
We consider the infrared modification of gravity by ghost condensate.
Naively, in this scenario one expects sizeable modification of gravity at
distances of order 1000 km, provided that the characteristic time scale of the
theory is of the order of the Hubble time. However, we argue that this is not
the case. The main physical reason for the conspiracy is a simple fact that the
Earth (and any other object in the Universe) has velocity of at least of order
10^{-3}c with respect to the rest frame of ghost condensate. Combined with
strong retardation effects present in the ghost sector, this fact implies that
no observable modification of the gravitational field of nearby objects occurs.
Instead, the physical manifestation of ghost condensate is the presence of
``star tracks'' -- narrow regions of space with growing gravitational and ghost
fields inside -- along the trajectory of any massive object. We briefly discuss
the possibilities to observe these tracks.Comment: 20 pages, 2 figures, final version published in JCA
The Accelerated Universe and the Moon
Cosmologically motivated theories that explain small acceleration rate of the
Universe via modification of gravity at very large, horizon or super-horizon
distances, can be tested by precision gravitational measurements at much
shorter scales, such as the Earth-Moon distance. Contrary to the naive
expectation the predicted corrections to the Einsteinian metric near
gravitating sources are so significant that fall within sensitivity of the
proposed Lunar Ranging experiments. The key reason for such corrections is the
van Dam-Veltman-Zakharov discontinuity present in linearized versions of all
such theories, and its subsequent absence at the non-linear level ala
Vainshtein
NMR and Mossbauer study of spin dynamics and electronic structure of Fe{2+x}V{1-x}Al and Fe2VGa
In order to assess the magnetic ordering process in Fe2VAl and the related
material Fe2VGa, we have carried out nuclear magnetic resonance (NMR) and
Mossbauer studies. 27Al NMR relaxation measurements covered the temperature
range 4 -- 500 K in Fe(2+x)V(1-x)Al samples. We found a peak in the NMR
spin-lattice relaxation rate, 27T1^-1, corresponding to the magnetic
transitions in each of these samples. These peaks appear at 125 K, 17 K, and
165 K for x = 0.10, 0, and - 0.05 respectively, and we connect these features
with critical slowing down of the localized antisite defects. Mossbauer
measurements for Fe2VAl and Fe2VGa showed lines with no hyperfine splitting,
and isomer shifts nearly identical to those of the corresponding sites in Fe3Al
and Fe3Ga, respectively. We show that a model in which local band filling leads
to magnetic regions in the samples, in addition to the localized antisite
defects, can account for the observed magnetic ordering behavior.Comment: 5 pages, 3 figure
Thermal properties of Lu 5 Ir 4 Si 10 near the charge-density-wave transition
We report the investigations of specific heat, thermal conductivity, as well as thermoelectric power on the charge-density-wave ͑CDW͒ compound Lu 5 Ir 4 Si 10 as a function of temperature. All thermal measurements consistently exhibit anomalous features around the CDW transition temperature T o ϳ80 K. Although the observations can be associated with the CDW formation, the measured anomalies are significantly large, in contrast to those in weak-coupled CDW materials. A quantitative analysis for the specific-heat data near the fluctuation region yields a critical exponent ␣ϳ2, much larger than the predicted value ␣ϭ0.5 in the extended mean-field theory assuming three-dimensional fluctuations. We also obtained a ratio ␥*/␥ϭ8.4, a factor of 6 larger than the BCS value 1.43 in the weak-coupling limit, indicating a strong coupling of this phase transition. Besides, the observed giant excess specific heat ⌬C p /C p ϳ26% and thermal conductivity ⌬/ϳ15% at T o further support this strong-coupling scenario. These large enhancements in C p and are attributed to the results of substantially thermal excitation and heat carried by the soft phonons at the transition. In addition, a rapid change in the sign of thermoelectric power at T o was observed, which provides a better understanding of the evolution of electronic band structure of the system below and above the CDW formation
Exponential Metric Fields
The Laser Interferometer Space Antenna (LISA) mission will use advanced
technologies to achieve its science goals: the direct detection of
gravitational waves, the observation of signals from compact (small and dense)
stars as they spiral into black holes, the study of the role of massive black
holes in galaxy evolution, the search for gravitational wave emission from the
early Universe. The gravitational red-shift, the advance of the perihelion of
Mercury, deflection of light and the time delay of radar signals are the
classical tests in the first order of General Relativity (GR). However, LISA
can possibly test Einstein's theories in the second order and perhaps, it will
show some particular feature of non-linearity of gravitational interaction. In
the present work we are seeking a method to construct theoretical templates
that limit in the first order the tensorial structure of some metric fields,
thus the non-linear terms are given by exponential functions of gravitational
strength. The Newtonian limit obtained here, in the first order, is equivalent
to GR.Comment: Accepted for publication in Astrophysics and Space Science, 17 page
Introduction to half-metallic Heusler alloys: Electronic Structure and Magnetic Properties
Intermetallic Heusler alloys are amongst the most attractive half-metallic
systems due to the high Curie temperatures and the structural similarity to the
binary semiconductors. In this review we present an overview of the basic
electronic and magnetic properties of both Heusler families: the so-called
half-Heusler alloys like NiMnSb and the the full-Heusler alloys like
CoMnGe. \textit{Ab-initio} results suggest that both the electronic and
magnetic properties in these compounds are intrinsically related to the
appearance of the minority-spin gap. The total spin magnetic moment
scales linearly with the number of the valence electrons , such that
for the full-Heusler and for the half-Heusler alloys,
thus opening the way to engineer new half-metallic alloys with the desired
magnetic properties.Comment: 28 pages, submitted for a special issue of 'Journal of Physics D:
Applied Physics' on Heusler alloy
Thermomagnetic hysteresis effects in NiMn and NiNnPd thin films
dc magnetization measurements, for zero-field cooled (MZFC) and field-cooled (MFC) cases, have been carried out for flash-evaporated Pd-doped NiMn thin films. These included reentrant phases (Ni76xPdx)Mn24, for 0x5, and Ni75Mn23Pd2, a pure spinglass phase. The studies were performed over the temperature range 3–300 K. Low-field magnetization measurements show the irreversibility effect (MZFC and MFC diverge at temperatures below the Curie temperature Tc . In Ni75Mn23Pd2, MZFC falls below MFC, as usually observed. However, in reentrant compositions, M ZFC crosses M FC upon warming into the ferromagnetic regime, where it stays above M FC at temperatures below Tc . This unusual behavior is attributed to a model of Imry and Ma in which, in a ferromagnet with antiferromagnetic impurities, the impurities can couple to the host ferromagnetic alignment and force the system to break into domains antiferromagnetically coupled to each other. Field-cooled hysteresis measurements indicate the uniaxial anisotropy in these samples to be small, in contrast with the rigid uniaxial anisotropy reported for the corresponding polycrystalline bulk samples. Since the lattice-orbit coupling is weak in the amorphous phase, this clearly demonstrates that the physical origin of the unidirectional anisotropy is associated with the spin-orbit coupling
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