984 research outputs found
Phase and group velocity tracing analysis of projected wave packet motion along oblique radar beams ? qualitative analysis of QP echoes
International audienceThe wave packets of atmospheric gravity waves were numerically generated, with a given characteristic wave period, horizontal wave length and projection mean wind along the horizontal wave vector. Their projection phase and group velocities along the oblique radar beam (vpr and vgr), with different zenith angle ? and azimuth angle ?, were analyzed by the method of phase- and group-velocity tracing. The results were consistent with the theoretical calculations derived by the dispersion relation, reconfirming the accuracy of the method of analysis. The RTI plot of the numerical wave packets were similar to the striation patterns of the QP echoes from the FAI irregularity region. We propose that the striation range rate of the QP echo is equal to the radial phase velocity vpr, and the slope of the energy line across the neighboring striations is equal to the radial group velocity vgr of the wave packet; the horizontal distance between two neighboring striations is equal to the characteristic wave period ?. Then, one can inversely calculate all the properties of the gravity wave responsible for the appearance of the QP echoes. We found that the possibility of some QP echoes being generated by the gravity waves originated from lower altitudes cannot be ruled out
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
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
Cosmological Models and Latest Observational Data
In this note, we consider the observational constraints on some cosmological
models by using the 307 Union type Ia supernovae (SNIa), the 32 calibrated
Gamma-Ray Bursts (GRBs) at , the updated shift parameter from WMAP
5-year data (WMAP5), and the distance parameter of the measurement of the
baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red
galaxies with the updated scalar spectral index from WMAP5. The tighter
constraints obtained here update the ones obtained previously in the
literature.Comment: 10 pages, 5 figures, 1 table, revtex4; v2: discussions added,
accepted by Eur. Phys. J. C; v3: published versio
Constraints on alternative models to dark energy
The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. This may be
the evidence of the breakdown of the standard Friemann equation. We consider a
general modified Friedmann equation. Three different models are analyzed in
detail. The current supernovae data and the Wilkinson microwave anisotropy
probe data are used to constrain these models. A detailed analysis of the
transition from the deceleration phase to the acceleration phase is also
performed.Comment: 10 pages, 1 figure, revtex
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 ÍCDWÍ 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 Ïł80 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 âŁÏł2, much larger than the predicted value âŁÏ0.5 in the extended mean-field theory assuming three-dimensional fluctuations. We also obtained a ratio â„*/â„Ï8.4, 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 âŹC p /C p Ïł26% and thermal conductivity âŹ/Ïł15% 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
Structure and magnetic order in Fe2+xV1-xAl
We present a detailed structural investigation via neutron diffraction of
differently heat treated samples Fe2VAl and Fe2+xV1-xAl. Moreover, the magnetic
behaviour of these materials is studied by means of mSR and
Mossbauer-experiments. Our structural investigation indicates that quenched
Fe2VAl, exhibiting the previously reported "Kondo insulating like" behaviour,
is off-stoichiometric (6%) in its Al content. Slowly cooled Fe2VAl is
structurally better ordered and stoichiometric, and the microscopic magnetic
probes establish long range ferromagnetic order below TC = 13K, consistent with
results from bulk experiments. The magnetic state can be modelled as being
generated by diluted magnetic ions in a non-magnetic matrix. Quantitatively,
the required number of magnetic ions is too large as to be explained by a model
of Fe/V site exchange. We discuss the implications of our findings for the
ground state properties of Fe2VAl, in particular with respect to the role of
crystallographic disorder.Comment: accepted for publication in J. Phys.: Condens. Matte
Cosmic Strings in a Braneworld Theory with Metastable Gravitons
If the graviton possesses an arbitrarily small (but nonvanishing) mass,
perturbation theory implies that cosmic strings have a nonzero Newtonian
potential. Nevertheless in Einstein gravity, where the graviton is strictly
massless, the Newtonian potential of a cosmic string vanishes. This discrepancy
is an example of the van Dam--Veltman--Zakharov (VDVZ) discontinuity. We
present a solution for the metric around a cosmic string in a braneworld theory
with a graviton metastable on the brane. This theory possesses those features
that yield a VDVZ discontinuity in massive gravity, but nevertheless is
generally covariant and classically self-consistent. Although the cosmic string
in this theory supports a nontrivial Newtonian potential far from the source,
one can recover the Einstein solution in a region near the cosmic string. That
latter region grows as the graviton's effective linewidth vanishes (analogous
to a vanishing graviton mass), suggesting the lack of a VDVZ discontinuity in
this theory. Moreover, the presence of scale dependent structure in the metric
may have consequences for the search for cosmic strings through gravitational
lensing techniques.Comment: 18 pages, 2 figures, revtex. Improved discussion of interpolating
solution. To be published in Phys. Rev.
- âŠ