27,004 research outputs found
An algebraic formula for the index of a vector field on an isolated complete intersection singularity
Let (V,0) be a germ of a complete intersection variety in \CC^{n+k}, n>0,
having an isolated singularity at 0 and X be the germ of a holomorphic vector
field on \CC^{n+k} tangent to V and having on V an isolated zero at 0. We show
that in this case the homological index and the GSV-index coincide. In the case
when the zero of X is also isolated in the ambient space \CC^{n+k} we give a
formula for the homological index in terms of local linear algebra.Comment: 18 pages; added an example which is not quasi homogeneous. A script
calculating this example can be found at
http://www.iag.uni-hannover.de/~bothmer/gobelin/ or at the and of the source
file of this articl
Response of a particle in a one-dimensional lattice to an applied force: Dynamics of the effective mass
We study the behaviour of the expectation value of the acceleration of a
particle in a one-dimensional periodic potential when an external homogeneous
force is suddenly applied. The theory is formulated in terms of modified Bloch
states that include the interband mixing induced by the force. This approach
allows us to understand the behaviour of the wavepacket, which responds with a
mass that is initially the bare mass, and subsequently oscillates around the
value predicted by the effective mass. If Zener tunneling can be neglected, the
expression obtained for the acceleration of the particle is valid over
timescales of the order of a Bloch oscillation, which are of interest for
experiments with cold atoms in optical lattices. We discuss how these
oscillations can be tuned in an optical lattice for experimental detection.Comment: 15 pages, 12 figure
Recommended from our members
Electrospray synthesis of PLGA TIPS microspheres
We successfully demonstrate the synthesis of polymer microspheres using a single electrospray source, and show their physical characterisation. Electrospray has proven to be a versatile method to manufacture particles, giving tight control over size with quasi-monodisperse size distributions. It is a liquid atomisation technique that generates a monodisperse population of highly charged liquid droplets over a broad size range (nanometres to tens of microns). The droplets contain liquid precursors for the in-flight synthesis of particles, and control over the trajectory of these droplets can be precisely manipulated with the use of electric fields to drive them to a grounded substrate. This study reports a method to synthesize poly(lactic-co-glycolic) acid (PLGA) microspheres using the electrospray and thermally induced phase separation (TIPS) techniques, followed by subsequent freeze-drying, for particle production. These microspheres are of interest as vehicles for controlled drug release systems
Specific Heat of the Dilute Ising Magnet LiHoYF
We present specific heat data on three samples of the dilute Ising magnet
\HoYLF with , 0.045 and 0.080. Previous measurements of the ac
susceptibility of an sample showed the Ho moments to remain
dynamic down to very low temperatures and the specific heat was found to have
unusually sharp features. In contrast, our measurements do not exhibit these
sharp features in the specific heat and instead show a broad feature, for all
three samples studied, which is qualitatively consistent with a spin glass
state. Integrating , however, reveals an increase in residual entropy with
lower Ho concentration, consistent with recent Monte Carlo simulations showing
a lack of spin glass transition for low x.Comment: 10 pages, 3 figurs, accepted for publication in Phys. Rev. Let
Controlling the quality factor of a tuning-fork resonance between 9 K and 300 K for scanning-probe microscopy
We study the dynamic response of a mechanical quartz tuning fork in the
temperature range from 9 K to 300 K. Since the quality factor Q of the
resonance strongly depends on temperature, we implement a procedure to control
the quality factor of the resonance. We show that we are able to dynamically
change the quality factor and keep it constant over the whole temperature
range. This procedure is suitable for applications in scanning probe
microscopy.Comment: 5 pages, 6 figure
A note on the likelihood and moments of the skew-normal distribution
In this paper an alternative approach to the one in Henze (1986) is proposed for deriving the odd moments of the skew-normal distribution considered in Azzalini (1985). The approach is based on a Pascal type triangle, which seems to greatly simplify moments computation. Moreover, it is shown that the likelihood equation for estimating the asymmetry parameter in such model is generated as orthogonal functions to the sample vector. As a consequence, conditions for a unique solution of the likelihood equation are established, which seem to hold in more general setting
Scaling in the time-dependent failure of a fiber bundle with local load sharing
We study the scaling behaviors of a time-dependent fiber-bundle model with
local load sharing. Upon approaching the complete failure of the bundle, the
breaking rate of fibers diverges according to ,
where is the lifetime of the bundle, and is a quite
universal scaling exponent. The average lifetime of the bundle scales
with the system size as , where depends on the
distribution of individual fiber as well as the breakdown rule.Comment: 5 pages, 4 eps figures; to appear in Phys. Rev.
The Herschel exploitation of local galaxy Andromeda (HELGA) V: Strengthening the case for substantial interstellar grain growth
In this paper we consider the implications of the distributions of dust and
metals in the disc of M31. We derive mean radial dust distributions using a
dust map created from Herschel images of M31 sampling the entire far-infrared
(FIR) peak. Modified blackbodies are fit to approximately 4000 pixels with a
varying, as well as a fixed, dust emissivity index (beta). An overall metal
distribution is also derived using data collected from the literature. We use a
simple analytical model of the evolution of the dust in a galaxy with dust
contributed by stellar sources and interstellar grain growth, and fit this
model to the radial dust-to-metals distribution across the galaxy. Our analysis
shows that the dust-to-gas gradient in M31 is steeper than the metallicity
gradient, suggesting interstellar dust growth is (or has been) important in
M31. We argue that M31 helps build a case for cosmic dust in galaxies being the
result of substantial interstellar grain growth, while the net dust production
from stars may be limited. We note, however, that the efficiency of dust
production in stars, e.g., in supernovae (SNe) ejecta and/or stellar
atmospheres, and grain destruction in the interstellar medium (ISM) may be
degenerate in our simple model. We can conclude that interstellar grain growth
by accretion is likely at least as important as stellar dust production
channels in building the cosmic dust component in M31.Comment: 12 pages, 7 figures. Published in MNRAS 444, 797. This version is
updated to match the published versio
H2O Maser Observations of Candidate Post-AGB Stars and Discovery of Three High-velocity Water Sources
We present the results of 22 GHz H_2O maser observations of a sample of 85
post-Asymptotic Giant Branch (post-AGB) candidate stars, selected on the basis
of their OH 1612 MHz maser and far-infrared properties. All sources were
observed with the Tidbinbilla 70-m radio telescope and 21 detections were made.
86 GHz SiO Mopra observations of a subset of the sample are also presented. Of
the 21 H_2O detections, 15 are from sources that are likely to be massive AGB
stars and most of these show typical, regular H_2O maser profiles. In contrast,
nearly all the detections of more evolved stars exhibited high-velocity H_2O
maser emission. Of the five sources seen, v223 (W43A, IRAS 18450-0148) is a
well known `water-fountain' source which belongs to a small group of post-AGB
stars with highly collimated, high-velocity H_2O maser emission. A second
source in our sample, v270 (IRAS 18596+0315), is also known to have
high-velocity emission. We report the discovery of similar emission from a
further three sources, d46 (IRAS 15445-5449), d62 (IRAS 15544-5332) and b292
(IRAS 18043-2116). The source d46 is an evolved post-AGB star with highly
unusual maser properties. The H_2O maser emission from d62 is probably
associated with a massive star. The source b292 is a young post-AGB star that
is highly likely to be a water-fountain source, with masers detected over a
velocity range of 210 km s^{-1}.Comment: 47 pages, 9 figures, 4 tables, accepted by Ap
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