19,041 research outputs found
Local density of states in metal - topological superconductor hybrid systems
We study by means of the recursive Green's function technique the local
density-of-states of (finite and semi-infinite) multi-band spin-orbit coupled
semiconducting nanowires in proximity to an s-wave superconductor and attached
to normal-metal electrodes. When the nanowire is coupled to a normal electrode,
the zero-energy peak, corresponding to the Majorana state in the topological
phase, broadens with increasing transmission between the wire and the leads,
eventually disappearing for ideal interfaces. Interestingly, for a finite
transmission a peak is present also in the normal electrode, even though it has
a smaller amplitude and broadens more rapidly with the strength of the
coupling. Unpaired Majorana states can survive close to a topological phase
transition even when the number of open channels (defined in the absence of
superconductivity) is even. We finally study the Andreev-bound-state spectrum
in superconductor-normal metal-superconductor junctions and find that in
multi-band nanowires the distinction between topologically trivial and
non-trivial systems based on the number of zero-energy crossings is preserved.Comment: 11 pages, 12 figures, published versio
Polynomial growth of volume of balls for zero-entropy geodesic systems
The aim of this paper is to state and prove polynomial analogues of the
classical Manning inequality relating the topological entropy of a geodesic
flow with the growth rate of the volume of balls in the universal covering. To
this aim we use two numerical conjugacy invariants, the {\em strong polynomial
entropy } and the {\em weak polynomial entropy }. Both are
infinite when the topological entropy is positive and they satisfy
. We first prove that the growth rate of the volume of
balls is bounded above by means of the strong polynomial entropy and we show
that for the flat torus this inequality becomes an equality. We then study the
explicit example of the torus of revolution for which we can give an exact
asymptotic equivalent of the growth rate of volume of balls, which we relate to
the weak polynomial entropy.Comment: 22 page
SwSt 1: an O-rich planetary nebula around a C-rich central star
The hydrogen-deficient carbon-rich [WCL] type central star HD167362 and its
oxygen-rich planetary nebula (PN) SwSt~1 are investigated. The nebular
chemistry might indicate a recent origin for the carbon-rich stellar spectrum.
Its stellar and nebular properties might therefore provide further
understanding of the origin of the [WCL] central star class. The UV-IR stellar
spectra are modelled with state of the codes and show ~40kK central star with a
wind and a C/O~3, indicative of efficient third dredge-up. The synthetic
stellar flux distribution is used to model the high density, compact PN, which
has a solar C/O ratio, is still enshrouded by 1200K and 230K dust shells and,
reported here for the first time, in molecular hydrogen. Although it appears
that the change in C/O ratio has been recent, the published spectroscopy since
1895 has been re-examined and no clear spectral change is seen. If an event
occurred that has turned it into a hydrogen-deficient central star, it did not
happen in the last 100 years.Comment: 31 pages, 19 figures (some are gif files), MNRAS in pres
Wolf-Rayet nebulae as tracers of stellar ionizing fluxes: I. M1-67
We use WR124 (WN8h) and its associated nebula M1-67, to test theoretical
non-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing flux
distributions derived from a stellar analysis of WR124, are compared with
nebular properties via photo-ionization modelling. Our study demonstrates the
significant role that line blanketing plays in affecting the Lyman ionizing
energy distribution of WR stars, of particular relevance to the study of HII
regions containing young stellar populations.
We confirm previous results that non-line blanketed WR energy distributions
fail to explain the observed nebular properties of M1-67, such that the
predicted ionizing spectrum is too hard. A line blanketed analysis of WR124 is
carried out using the method of Hillier & Miller (1998), with stellar
properties in accord with previous results, except that the inclusion of
clumping in the stellar wind reduces its wind performance factor to only
approx2. The ionizing spectrum of the line blanketed model is much softer than
for a comparable temperature unblanketed case, such that negligible flux is
emitted with energy above the HeI 504 edge. Photo-ionization modelling,
incorporating the observed radial density distribution for M1-67 reveals
excellent agreement with the observed nebular electron temperature, ionization
balance and line strengths. An alternative stellar model of WR124 is
calculated, following the technique of de Koter et al. (1997), augmented to
include line blanketing following Schmutz et al. (1991). Good consistency is
reached regarding the stellar properties of WR124, but agreement with the
nebular properties of M1-67 is somewhat poorer than for the Hillier & Miller
code.Comment: 12 pages, 5 figures, latex2e style file, Astronomy & Astrophysics
(accepted
Mixing-scale dependent dispersion for transport in heterogeneous flows
Dispersion quantifies the impact of subscale velocity fluctuations on the effective movement of particles and the evolution of scalar distributions in heterogeneous flows. Which fluctuation scales are represented by dispersion, and the very meaning of dispersion, depends on the definition of the subscale, or the corresponding coarse-graining scale. We study here the dispersion effect due to velocity fluctuations that are sampled on the homogenization scale of the scalar distribution. This homogenization scale is identified with the mixing scale, the characteristic length below which the scalar is well mixed. It evolves in time as a result of local-scale dispersion and the deformation of material fluid elements in the heterogeneous flow. The fluctuation scales below the mixing scale are equally accessible to all scalar particles, and thus contribute to enhanced scalar dispersion and mixing. We focus here on transport in steady spatially heterogeneous flow fields such as porous media flows. The dispersion effect is measured by mixing-scale dependent dispersion coefficients, which are defined through a filtering operation based on the evolving mixing scale. This renders the coarse-grained velocity as a function of time, which evolves as velocity fluctuation scales are assimilated by the expanding scalar. We study the behaviour of the mixing-scale dependent dispersion coefficients for transport in a random shear flow and in heterogeneous porous media. Using a stochastic modelling framework, we derive explicit expressions for their time behaviour. The dispersion coefficients evolve as the mixing scale scans through the pertinent velocity fluctuation scales, which reflects the fundamental role of the interaction of scalar and velocity fluctuation scales in solute mixing and dispersion. © © 2015 Cambridge University Press.The authors thank three anonymous reviewers for their insightful comments. M.D.
acknowledges the support of the European Research Council (ERC) through the
project MHetScale (617511).Peer reviewe
A Spitzer view of protoplanetary disks in the gamma Velorum cluster
We present new Spitzer Space Telescope observations of stars in the young ~5
Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we
have selected 579 stars as candidate members of the cluster. With the addition
of the Spitzer mid-infrared data, we have identified 5 debris disks around
A-type stars, and 5-6 debris disks around solar-type stars, indicating that the
strong radiation field in the cluster does not completely suppress the
production of planetesimals in the disks of cluster members. However, we find
some evidence that the frequency of circumstellar primordial disks is lower,
and the IR flux excesses are smaller than for disks around stellar populations
with similar ages. This could be evidence for a relatively fast dissipation of
circumstellar dust by the strong radiation field from the highest mass star(s)
in the cluster. Another possibility is that gamma Velorum stellar cluster is
slightly older than reported ages and the the low frequency of primordial disks
reflects the fast disk dissipation observed at ~5 Myr.Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11
Figure
ENOBIO - First tests of a dry electrophysiology electrode using carbon nanotubes
We describe the development and first tests of Enobio, a dry electrode sensor
concept for biopotential applications. In the proposed electrodes, the tip of
the electrode is covered with a forest of multi-walled CNTs that can be coated
with Ag/AgCl to provide ionic-electronic transduction. The CNT brush-like
structure is to penetrate the outer layers of the skin improving electrical
contact as well as increae the contact surface area. In this paper, we report
the results of the first tests of this concept -- immersion on saline solution
and pig skin signal detection. These indicate performance on a par with state
of the art research-oriented wet electrodes.Comment: Submitted and accepted at the 28th IEEE EMBS International
Conference, New York City, August 31st-September 3rd, 2006. Figures updated
with proper filtering and averagin
Have Cherenkov telescopes detected a new light boson?
Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe
is more transparent to very-high-energy gamma rays than previously thought. We
show that this fact can be reconciled with standard blazar emission models
provided that photon oscillations into a very light Axion-Like Particle occur
in extragalactic magnetic fields. A quantitative estimate of this effect indeed
explains the observed data and in particular the spectrum of blazar 3C279.Comment: 3 pages, 1 figure, Proceeding of the "Eleventh International Workshop
on Topics in Astroparticle and Underground Physics" (TAUP), Roma, Italy, 1 -
5 July 2009 (to be published in the Proceedings
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