911 research outputs found
Multicolour photometry of Balloon 090100001: linking the two classes of pulsating hot subdwarfs
We present results of the multicolour UBVR photometry of the high-amplitude
EC14026-type star, Balloon 090100001. The data span over a month and consist of
more than a hundred hours of observations. Fourier analysis of these data led
us to the detection of at least 30 modes of pulsation of which 22 are
independent. The frequencies of 13 detected modes group in three narrow ranges,
around 2.8, 3.8 and 4.7 mHz, where the radial fundamental mode, the first and
second overtones are likely to occur. Surprisingly, we also detect 9
independent modes in the low-frequency domain, between 0.15 and 0.4 mHz. These
modes are typical for pulsations found in PG1716+426-type stars, discovered
recently among cool B-type subdwarfs. The modes found in these stars are
attributed to the high-order g modes. As both kinds of pulsations are observed
in Balloon 090100001, it represents a link between the two classes of pulsating
hot subdwarfs. At present, it is probably the most suitable target for testing
evolutionary scenarios and internal constitution models of these stars by means
of asteroseismology.
Three of the modes we discovered form an equidistant frequency triplet which
can be explained by invoking rotational splitting of an = 1 mode. The
splitting amounts to about 1.58 Hz, leading to a rotation period of 7.1
0.1 days.Comment: 12 pages, 14 figures, accepted for publication in MNRAS. For
full-resolution postscript file, visit
http://www.as.wsp.krakow.pl/~andy/balloon.ps.g
Nanometric pitch in modulated structures of twist-bend nematic liquid crystals
The extended Frank elastic energy density is used to investigate the
existence of a stable periodically modulate structure that appears as a ground
state exhibiting a twist-bend molecular arrangement. For an unbounded sample,
we show that the twist-bend nematic phase is characterized by a
heliconical structure with a pitch in the nano-metric range, in agreement with
experimental results. For a sample of finite thickness, we show that the wave
vector of the stable periodic structure depends not only on the elastic
parameters but also on the anchoring energy, easy axis direction, and the
thickness of the sample.Comment: 11 page
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
Photometric, Spectroscopic and Orbital Period Study of Three Early Type Semi-detached Systems: XZ Aql, UX Her and AT Peg
In this paper we present a combined photometric, spectroscopic and orbital
period study of three early-type eclipsing binary systems: XZ Aql, UX Her, and
AT Peg. As a result, we have derived the absolute parameters of their
components and, on that basis, we discuss their evolutionary states.
Furthermore, we compare their parameters with those of other binary systems and
with the theoretical models. An analysis of all available up-to-date times of
minima indicated that all three systems studied here show cyclic orbital
changes, their origin is discussed in detail. Finally, we performed a frequency
analysis for possible pulsational behavior and as a result we suggest that XZ
Aql hosts a {\delta} Scuti component.Comment: 40 pages, 16 figure
Estimating physical properties from liquid crystal textures via machine learning and complexity-entropy methods
Imaging techniques are essential tools for inquiring a number of properties
from different materials. Liquid crystals are often investigated via optical
and image processing methods. In spite of that, considerably less attention has
been paid to the problem of extracting physical properties of liquid crystals
directly from textures images of these materials. Here we present an approach
that combines two physics-inspired image quantifiers (permutation entropy and
statistical complexity) with machine learning techniques for extracting
physical properties of nematic and cholesteric liquid crystals directly from
their textures images. We demonstrate the usefulness and accuracy of our
approach in a series of applications involving simulated and experimental
textures, in which physical properties of these materials (namely: average
order parameter, sample temperature, and cholesteric pitch length) are
predicted with significant precision. Finally, we believe our approach can be
useful in more complex liquid crystal experiments as well as for probing
physical properties of other materials that are investigated via imaging
techniques.Comment: 11 two-column pages, 7 figures; accepted for publication in Physical
Review
Study of coupling loss on bi-columnar BSCCO/Ag tapes by a.c. susceptibility measurements
Coupling losses were studied in composite tapes containing superconducting
material in the form of two separate stacks of densely packed filaments
embedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometry
is quite favorable for studying the coupling currents and in particular the
role of superconducting bridges between filaments. By using a.c. susceptibility
technique, the electromagnetic losses as function of a.c. magnetic field
amplitude and frequency were measured at the temperature T = 77 K for two tapes
with different matrix composition. The length of samples was varied by
subsequent cutting in order to investigate its influence on the dynamics of
magnetic flux penetration. The geometrical factor which takes into
account the demagnetizing effects was established from a.c. susceptibility data
at low amplitudes. Losses vs frequency dependencies have been found to agree
nicely with the theoretical model developed for round multifilamentary wires.
Applying this model, the effective resistivity of the matrix was determined for
each tape, by using only measured quantities. For the tape with pure silver
matrix its value was found to be larger than what predicted by the theory for
given metal resistivity and filamentary architecture. On the contrary, in the
sample with a Ag/Mg alloy matrix, an effective resistivity much lower than
expected was determined. We explain these discrepancies by taking into account
the properties of the electrical contact of the interface between the
superconducting filaments and the normal matrix. In the case of soft matrix of
pure Ag, this is of poor quality, while the properties of alloy matrix seem to
provoke an extensive creation of intergrowths which can be actually observed in
this kind of samples.Comment: 20 pages 11 figure, submitted to Superconductor Science and
Technolog
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