19,840 research outputs found
Supercurrent on a vortex core in 2H-NbSe: current driven scanning tunneling spectroscopy
We report current driven scanning tunneling spectroscopy (CDSTS) measurements
at very low temperatures on vortices in 2H-NbSe2. We find that a current
produces an increase of the density of states at the Fermi level in between
vortices, and a reduction of the zero bias peak at the vortex center. This
occurs well below the de-pairing current. We conclude that a supercurrent
affects the low energy part of the superconducting gap structure of 2H-NbSe2.Comment: 5 pages, 5 figure
Stabilized jellium model and structural relaxation effects on the fragmentation energies of ionized silver clusters
Using the stabilized jellium model in two schemes of `relaxed' and `rigid',
we have calculated the dissociation energies and the fission barrier heights
for the binary fragmentations of singly-ionized and doubly-ionized Ag clusters.
In the calculations, we have assumed spherical geometries for the clusters.
Comparison of the fragmentation energies in the two schemes show differences
which are significant in some cases. This result reveals the advantages of the
relaxed SJM over the rigid SJM in dynamical processes such as fragmentation.
Comparing the relaxed SJM results and axperimental data on fragmentation
energies, it is possible to predict the sizes of the clusters just before their
fragmentations.Comment: 9 pages, 12 JPG figure
Scalar fields in the Lense-Thirring background with a cosmic string and Hawking radiation
We analyze the influence of the gravitational field produced by a slowly
rotating black hole with a cosmic string along the axis of symmetry on a
massive scalar field. Exact solutions of both angular and radial parts of the
Klein-Gordon equation in this spacetime are obtained, and are given in terms of
the confluent Heun functions. We emphasize the role of the presence of the
cosmic string in these solutions. We also investigate the solutions in regions
near and far from the event horizon. From the radial solution, we obtain the
exact wave solutions near the exterior horizon of the black hole, and discuss
the Hawking radiation of massive scalar particles.Comment: 6 page
The slimming effect of advection on black-hole accretion flows
At super-Eddington rates accretion flows onto black holes have been described
as slim (aspect ratio ) or thick (H/R >1) discs, also known as
tori or (Polish) doughnuts. The relation between the two descriptions has never
been established, but it was commonly believed that at sufficiently high
accretion rates slim discs inflate, becoming thick. We wish to establish under
what conditions slim accretion flows become thick. We use analytical equations,
numerical 1+1 schemes, and numerical radiative MHD codes to describe and
compare various accretion flow models at very high accretion rates.We find that
the dominant effect of advection at high accretion rates precludes slim discs
becoming thick. At super-Eddington rates accretion flows around black holes can
always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres
Modelling resonances and orbital chaos in disk galaxies. Application to a Milky Way spiral model
Context: Resonances in the stellar orbital motion under perturbations from
spiral arms structure play an important role in the evolution of the disks of
spiral galaxies. The epicyclic approximation allows the determination of the
corresponding resonant radii on the equatorial plane (for nearly circular
orbits), but is not suitable in general.
Aims: We expand the study of resonant orbits by analysing stellar motions
perturbed by spiral arms with Gaussian-shaped profiles without any restriction
on the stellar orbital configurations, and we expand the concept of Lindblad
(epicyclic) resonances for orbits with large radial excursions.
Methods: We define a representative plane of initial conditions, which covers
the whole phase space of the system. Dynamical maps on representative planes
are constructed numerically, in order to characterize the phase-space structure
and identify the precise location of resonances. The study is complemented by
the construction of dynamical power spectra, which provide the identification
of fundamental oscillatory patterns in the stellar motion.
Results: Our approach allows a precise description of the resonance chains in
the whole phase space, giving a broader view of the dynamics of the system when
compared to the classical epicyclic approach, even for objects in retrograde
motion. The analysis of the solar neighbourhood shows that, depending on the
current azimuthal phase of the Sun with respect to the spiral arms, a star with
solar kinematic parameters may evolve either inside the stable co-rotation
resonance or in a chaotic zone.
Conclusions: Our approach contributes to quantifying the domains of resonant
orbits and the degree of chaos in the whole Galactic phase-space structure. It
may serve as a starting point to apply these techniques to the investigation of
clumps in the distribution of stars in the Galaxy, such as kinematic moving
groups.Comment: 17 pages, 15 figures. Matches accepted version in A&
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