12,683 research outputs found
How to generate pentagonal symmetry using Turing systems
We explore numerically the formation of Turing patterns in a confined circular domain with small aspect ratio. Our results show that stable fivefold patterns are formed over a well defined range of disk sizes, offering a possible mechanism for inducing the fivefold symmetry observed in early development of regular echinoids. Using this pattern as a seed, more complex biological structures can be mimicked, such as the pigmentation pattern of sea urchins and the plate arrangements of the calyxes of primitive camerate crinoids
Weber blockade theory of magnetoresistance oscillations in superconducting strips
Recent experiments on the conductance of thin, narrow superconducting strips
have found periodic fluctuations, as a function of the perpendicular magnetic
field, with a period corresponding to approximately two flux quanta per strip
area [A. Johansson et al., Phys. Rev. Lett. {\bf 95}, 116805 (2005)]. We argue
that the low-energy degrees of freedom responsible for dissipation correspond
to vortex motion. Using vortex/charge duality, we show that the superconducting
strip behaves as the dual of a quantum dot, with the vortices, magnetic field,
and bias current respectively playing the roles of the electrons, gate voltage
and source-drain voltage. In the bias-current vs. magnetic-field plane, the
strip conductance displays what we term `Weber blockade' diamonds, with vortex
conductance maxima (i.e., electrical resistance maxima) that, at small
bias-currents, correspond to the fields at which strip states of and
vortices have equal energy.Comment: 4+a bit pages, 3 figures, 1 tabl
HS 2325+8205 - an ideal laboratory for accretion disk physics
We identify HS 2325+8205 as an eclipsing, frequently outbursting dwarf nova
with an orbital period of 279.841731(5) min. Spectroscopic observations are
used to derive the radial velocity curve of the secondary star from absorption
features and also from the H-alpha emission lines, originating from the
accretion disc, yielding K_secondary = K_abs = 237 +- 28 km/s and K_emn = 145
+- 9 km/s respectively. The distance to the system is calculated to be 400
(+200, -140) pc. A photometric monitoring campaign reveals an outburst
recurrence time of 12-14 d, The combination of magnitude range (17-14 mag),
high declination, eclipsing nature and frequency of outbursts makes HS
2325+8205 the ideal system for "real-time" studies of the accretion disc
evolution and behavior in dwarf nova outbursts.Comment: 20 pages, 7 figures. Accepted for Publications of the Astronomical
Society of the Pacifi
On the Transverse-Traceless Projection in Lattice Simulations of Gravitational Wave Production
It has recently been pointed out that the usual procedure employed in order
to obtain the transverse-traceless (TT) part of metric perturbations in lattice
simulations was inconsistent with the fact that those fields live in the
lattice and not in the continuum. It was claimed that this could lead to a
larger amplitude and a wrong shape for the gravitational wave (GW) spectra
obtained in numerical simulations of (p)reheating. In order to address this
issue, we have defined a consistent prescription in the lattice for extracting
the TT part of the metric perturbations. We demonstrate explicitly that the GW
spectra obtained with the old continuum-based TT projection only differ
marginally in amplitude and shape with respect to the new lattice-based ones.
We conclude that one can therefore trust the predictions appearing in the
literature on the spectra of GW produced during (p)reheating and similar
scenarios simulated on a lattice.Comment: 22 pages, 8 figures, Submitted to JCA
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap
Electronic and Thermoelectric Properties of RuIn_{3-x}A_{x} (A = Sn, Zn)
Recently, we reported [M. Wagner et al., J. Mater. Res. 26, 1886 (2011)]
transport measurements on the semiconducting intermetallic system RuIn3 and its
substitution derivatives RuIn_{3-x}A_{x} (A = Sn, Zn). Higher values of the
thermoelectric figure of merit (zT = 0.45) compared to the parent compound were
achieved by chemical substitution. Here, using density functional theory based
calculations, we report on the microscopic picture behind the measured
phenomenon. We show in detail that the electronic structure of the substitution
variants of the intermetallic system RuIn_{3-x}A_{x} (A = Sn, Zn) changes in a
rigid-band like fashion. This behavior makes possible the fine tuning of the
substitution concentration to take advantage of the sharp peak-like features in
the density of states of the semiconducting parent compound. Trends in the
transport properties calculated using the semi-classical Boltzmann transport
equations within the constant scattering time approximation are in good
agreement with the former experimental results for RuIn_{3-x}Sn_{x}. Based on
the calculated thermopower for the p-doped systems, we reinvestigated the
Zn-substituted derivative and obtained ZnO-free RuIn_{3-x}Zn_{x}. The new
experimental results are consistent with the calculated trend in thermopower
and yield large zT value of 0.8.Comment: PRB Accepted, 11 pages, 10 figure
An annular gap acceleration model for -ray emission of pulsars
If the binding energy of the pulsar's surface is not so high (the case of a
neutron star), both the negative and positive charges will flow out freely from
the surface of the star. The annular free flow model for -ray emission
of pulsars is suggested in this paper. It is emphasized that: (1). Two kinds of
acceleration regions (annular and core) need to be taken into account. The
annular acceleration region is defined by the magnetic field lines that cross
the null charge surface within the light cylinder. (2). If the potential drop
in the annular region of a pulsar is high enough (normally the cases of young
pulsars), charges in both the annular and the core regions could be accelerated
and produce primary gamma-rays. Secondary pairs are generated in both regions
and stream outwards to power the broadband radiations. (3). The potential drop
in the annular region grows more rapidly than that in the core region. The
annular acceleration process is a key point to produce wide emission beams as
observed. (4). The advantages of both the polar cap and outer gap models are
retained in this model. The geometric properties of the -ray emission
from the annular flow is analogous to that presented in a previous work by Qiao
et al., which match the observations well. (5). Since charges with different
signs leave the pulsar through the annular and the core regions, respectively,
the current closure problem can be partially solved.Comment: 11 pages 2 figures, accepted by Chinese Journal of Astronomy and
Astrophysic
Supernova Neutrino Oscillations
Observing a high-statistics neutrino signal from a galactic supernova (SN)
would allow one to test the standard delayed explosion scenario and may allow
one to distinguish between the normal and inverted neutrino mass ordering due
to the effects of flavor oscillations in the SN envelope. One may even observe
a signature of SN shock-wave propagation in the detailed time-evolution of the
neutrino spectra. A clear identification of flavor oscillation effects in a
water Cherenkov detector probably requires a megatonne-class experiment.Comment: Proc. 129 Nobel Symposium "Neutrino Physics", 19-24 Aug 2004, Swede
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