21,131 research outputs found
Accelerating universes driven by bulk particles
We consider our universe as a 3d domain wall embedded in a 5d dimensional
Minkowski space-time. We address the problem of inflation and late time
acceleration driven by bulk particles colliding with the 3d domain wall. The
expansion of our universe is mainly related to these bulk particles. Since our
universe tends to be permeated by a large number of isolated structures, as
temperature diminishes with the expansion, we model our universe with a 3d
domain wall with increasing internal structures. These structures could be
unstable 2d domain walls evolving to fermi-balls which are candidates to cold
dark matter. The momentum transfer of bulk particles colliding with the 3d
domain wall is related to the reflection coefficient. We show a nontrivial
dependence of the reflection coefficient with the number of internal dark
matter structures inside the 3d domain wall. As the population of such
structures increases the velocity of the domain wall expansion also increases.
The expansion is exponential at early times and polynomial at late times. We
connect this picture with string/M-theory by considering BPS 3d domain walls
with structures which can appear through the bosonic sector of a
five-dimensional supergravity theory.Comment: To appear in Phys. Rev. D, 16 pages, 3 eps figures, minor changes and
references adde
Cusp Disruption in Minor Mergers
We present 0.55 x 10^6 particle simulations of the accretion of high-density
dwarf galaxies by low-density giant galaxies, using models that contain both
power-law central density cusps and point masses representing supermassive
black holes. The cusp of the dwarf galaxy is disrupted during the merger,
producing a remnant with a central density that is only slightly higher than
that of the giant galaxy initially. Removing the black hole from the giant
galaxy allows the dwarf galaxy to remain intact and leads to a remnant with a
high central density, contrary to what is observed. Our results support the
hypothesis that the persistence of low-density cores in giant galaxies is a
consequence of supermassive black holes.Comment: 5 pages, 2 postscript figures, uses emulateapj.sty. Accepted for
publication in The Astrophysical Journal Letter
Gravity localization on hybrid branes
This work deals with gravity localization on codimension-1 brane worlds
engendered by compacton-like kinks, the so-called hybrid branes. In such
scenarios, the thin brane behaviour is manifested when the extra dimension is
outside the compact domain, where the energy density is non-trivial, instead of
asymptotically as in the usual thick brane models. The zero mode is trapped in
the brane, as required. The massive modes, although are not localized in the
brane, have important phenomenological implications such as corrections to the
Newton's law. We study such corrections in the usual thick domain wall and in
the hybrid brane scenarios. By means of suitable numerical methods, we attain
the mass spectrum for the graviton and the corresponding wavefunctions. The
spectra possess the usual linearly increasing behaviour from the Kaluza-Klein
theories. Further, we show that the 4D gravitational force is slightly
increased at short distances. The first eigenstate contributes highly for the
correction to the Newton's law. The subsequent normalized solutions have
diminishing contributions. Moreover, we find out that the phenomenology of the
hybrid brane is not different from the usual thick domain wall. The use of
numerical techniques for solving the equations of the massive modes is useful
for matching possible phenomenological measurements in the gravitational law as
a probe to warped extra dimensions.Comment: 15 pages, 11 figure
Orbital magnetism in axially deformed sodium clusters: From scissors mode to dia-para magnetic anisotropy
Low-energy orbital magnetic dipole excitations, known as scissors mode (SM),
are studied in alkali metal clusters. Subsequent dynamic and static effects are
explored. The treatment is based on a self-consistent microscopic approach
using the jellium approximation for the ionic background and the Kohn-Sham mean
field for the electrons. The microscopic origin of SM and its main features
(structure of the mode in light and medium clusters, separation into low- and
high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole
plasmons, contributions of shape isomers, etc) are discussed. The scissors M1
strength acquires large values with increasing cluster size. The mode is
responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum
shell effects induce a fragile interplay between Langevin diamagnetism and van
Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic
susceptibility of particular light clusters. Finally, several routes for
observing the SM experimentally are discussed.Comment: 21 pages, 7 figure
Discrepancies in Determinations of the Ginzburg-Landau Parameter
Long-standing discrepancies within determinations of the Ginzburg-Landau
parameter from supercritical field measurements on superconducting
microspheres are reexamined. The discrepancy in tin is shown to result from
differing methods of analyses, whereas the discrepancy in indium is a
consequence of significantly differing experimental results. The reanalyses
however confirms the lower determinations to within experimental
uncertainties.Comment: submitted to Phys. Rev.
Structural Anomalies at the Magnetic and Ferroelectric Transitions in (R=Tb, Dy, Ho)
Strong anomalies of the thermal expansion coefficients at the magnetic and
ferroelectric transitions have been detected in multiferroic . Their
correlation with anomalies of the specific heat and the dielectric constant is
discussed. The results provide evidence for the magnetic origin of the
ferroelectricity mediated by strong spin-lattice coupling in the compounds.
Neutron scattering data for indicate a spin reorientation at the
two low-temperature phase transitions
Pressure-Temperature Phase Diagram of Multiferroic
The pressure-temperature phase diagram of multiferroic is
investigated for hydrostatic pressures up to 2 GPa. The stability range of the
ferroelectric phase associated with the incommensurate helical spin order is
reduced by pressure and ferroelectricity is completely suppressed at the
critical pressure of 1.64 GPa at 6.2 K. Thermal expansion measurements at
ambient pressure show strong step-like anomalies of the lattice parameters
associated with the lock-in transition into the commensurate paraelectric
phase. The expansion anomalies are highly anisotropic, the related volume
change is consistent with the high-pressure phase diagram
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