57 research outputs found
Inhomogeneous scalar field solutions and inflation
We present new exact cosmological inhomogeneous solutions for gravity coupled
to a scalar field in a general framework specified by the parameter .
The equations of motion (and consequently the solutions) in this framework
correspond either to low-energy string theory or Weyl integrable spacetime
according to the sign of . We show that different inflationary
behaviours are possible, as suggested by the study of the violation of the
strong energy condition. Finally, by the analysis of certain curvature scalars
we found that some of the solutions may be nonsingular.Comment: LaTex file, 14 page
Trapping dust particles in the outer regions of protoplanetary disks
In order to explain grain growth to mm sized particles and their retention in
outer regions of protoplanetary disks, as it is observed at sub-mm and mm
wavelengths, we investigate if strong inhomogeneities in the gas density
profiles can slow down excessive radial drift and can help dust particles to
grow. We use coagulation/fragmentation and disk-structure models, to simulate
the evolution of dust in a bumpy surface density profile which we mimic with a
sinusoidal disturbance. For different values of the amplitude and length scale
of the bumps, we investigate the ability of this model to produce and retain
large particles on million years time scales. In addition, we introduced a
comparison between the pressure inhomogeneities considered in this work and the
pressure profiles that come from magnetorotational instability. Using the
Common Astronomy Software Applications ALMA simulator, we study if there are
observational signatures of these pressure inhomogeneities that can be seen
with ALMA. We present the favorable conditions to trap dust particles and the
corresponding calculations predicting the spectral slope in the mm-wavelength
range, to compare with current observations. Finally we present simulated
images using different antenna configurations of ALMA at different frequencies,
to show that the ring structures will be detectable at the distances of the
Taurus Auriga or Ophiucus star forming regions.Comment: Pages 15, Figures 14. Accepted for publication in Astronomy and
Astrophysic
On the back reaction of gravitational and particle emission and absorption from straight thick cosmic strings: A toy model
The emission and absorption of gravitational waves and massless particles of
an infinitely long straight cosmic string with finite thickness are studied. It
is shown in a general term that the back reaction of the emission and
absorption {\em always} makes the symmetry axis of the string singular. The
singularity is a scalar singularity and cannot be removed.Comment: To appear in Gen. Relativ. Gra
Magnetic Anisotropy of a Single Cobalt Nanoparticle
Using a new microSQUID set-up, we investigate magnetic anisotropy in a single
1000-atoms cobalt cluster. This system opens new fields in the characterization
and the understanding of the origin of magnetic anisotropy in such
nanoparticles. For this purpose, we report three-dimensional switching field
measurements performed on a 3 nm cobalt cluster embedded in a niobium matrix.
We are able to separate the different magnetic anisotropy contributions and
evidence the dominating role of the cluster surface.Comment: 4 pages, 8 figure
Tunneling Anisotropic Magnetoresistance in Co/AlOx/Au Tunnel Junctions
We observe spin-valve-like effects in nano-scaled thermally evaporated
Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and
depends on the relative orientation of the magnetization direction of the Co
electrode with respect to the current direction. We attribute this effect to a
two-step magnetization reversal and an anisotropic density of states resulting
from spin-orbit interaction. The results of this study points to future
applications of novel spintronics devices involving only one ferromagnetic
layer.Comment: 11 pages, 5 figures. Accpted for publishing on Nano Letters, 200
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