1,499 research outputs found
Axions and White Dwarfs
White dwarfs are almost completely degenerate objects that cannot obtain
energy from the thermonuclear sources and their evolution is just a
gravothermal process of cooling. The simplicity of these objects, the fact that
the physical inputs necessary to understand them are well identified, although
not always well understood, and the impressive observational background about
white dwarfs make them the most well studied Galactic population. These
characteristics allow to use them as laboratories to test new ideas of physics.
In this contribution we discuss the robustness of the method and its
application to the axion case.Comment: 4 pages, 1 figure, to appear in the Proceedings for the 6th Patras
meeting on Axions, WIMPs and WISP
Changes in the structure of the accretion disc of EX Draconis through the outburst cycle
(Abridged) We report on the analysis of high-speed photometry of the dwarf
nova EX Dra through its outburst cycle with eclipse mapping techniques. The
eclipse maps show evidence of the formation of a one-armed spiral structure in
the disc at the early stages of the outburst and reveal how the disc expands
during the rise until it fills most of the primary Roche lobe at maximum light.
The eclipse maps also suggest the presence of an inward and an outward-moving
heating wave during the rise and an inward-moving cooling wave in the decline.
Our results suggest a systematic deceleration of both the heating and the
cooling waves as they travel across the disc, in agreement with predictions of
the disc instability model. The analysis of the brightness temperature profiles
indicates that most of the disc appears to be in steady-state during quiescence
and at outburst maximum, but not during the intermediate stages. As a general
trend, the mass accretion rate in the outer regions is larger than in the inner
disc on the rising branch, while the opposite holds during the decline branch.
We estimate a mass accretion rate of Mdot= 10^{-8} Msun/yr at outburst maximum
and Mdot= 10^{-9.1} Msun/yr in quiescence. The brightness temperature profile
in quiescence also suggests that the viscosity parameter is high at this stage
which favours the mass-transfer instability model. The uneclipsed light has a
steady component, understood in terms of emission from the red secondary star,
and a variable component that is proportional to the out of eclipse flux and
corresponds to about 3 per cent of the total brightness of the system. The
variable component is interpreted as arising in a disc wind.Comment: To appear in Mon. Not. R. Ast. Soc., 14 pages, 9 figures, coded with
MNRAS latex styl
Origin Of the enhanced flexoelectricity of relaxor ferroelectrics
We have measured the bending-induced polarization of Pb(Mg1/3Nb2/3)O3-PbTiO3
single crystals with compositions at the relaxor-ferroelectric phase boundary.
The crystals display very large flexoelectricity, with flexocoupling
coefficients an order of magnitude bigger than the theoretical upper limit set
by the theories of Kogan and Tagantsev. This enhancement persists in the
paraphrase up to a temperature T* that coincides with the start of elastic
softening in the crystals. Analysis of the temperature dependence and
cross-correlation between flexoelectric, dielectric and elastic properties
indicates that the large bendinginduced polarization of relaxor ferroelectrics
is not caused by intrinsically giant flexoelectricity, but by the reorientation
of polar nanotwins that become ferroelastically active below T*
Multi-epoch Doppler tomography and polarimetry of QQ Vul
We present multi-epoch high-resolution spectroscopy and photoelectric polarimetry of the long-period polar (AM Herculis star) QQ Vul. The blue emission lines show several distinct components, the sharpest of which can unequivocally be assigned to the illuminated hemisphere of the secondary star and used to trace its orbital motion. This narrow emission line can be used in combination with Nai-absorption lines from the photosphere of the companion to build a stable long-term ephemeris for the star: inferior conjunction of the companion occurs at HJD = 244 8446.4710(5)+E×0. d 15452011(11). The polarization curves are dissimilar at different epochs, thus supporting the idea of fundamental changes of the accretion geometry, e.g. between one- and two-pole accretion modes. The linear polarization pulses display a random scatter by 0.2 phase units and are not suitable for the determination of the binary period. The polarization data suggest that the magnetic (dipolar) axis has a co-latitude of 23 ◦ , an azimuth of −50 ◦, and an orbital inclination between 50 ◦ and 70 ◦. Doppler images of blue emission and red absorption lines show a clear separatio
Effect of manganese doping on the size effect of lead zirconate titanate thin films and the extrinsic nature of dead layers
We have investigated the size effect in lead zirconate titanate (PZT) thin
films with a range of manganese (Mn) doping concentrations. We found that the
size effect in the conventional Pt/PZT/Pt thin-film capacitors could be
systematically reduced and almost completely eliminated by increasing Mn doping
concentration. The interfacial layer at the electrode-film interface appears to
disappear almost entirely for the PZT films with 2% Mn doping levels, confirmed
by the fits using the conventional in-series capacitor model. Our work
indicates that the size effect in ferroelectrics is extrinsic in nature,
supporting the work by Saad et al. Other implications of our results have also
been discussed. By comparing a variety of experimental studies in the
literature we propose a scenario that the dead layer between PZT (or barium
strontium titanate, BST) and metal electrodes such as Pt and Au might have a
defective pyrochlore/fluorite structure (possibly with a small portion of
ferroelectric perovskite phase).Comment: 21 pages, 6 figure
Fractal dimension and size scaling of domains in thin films of multiferroic BiFeO3
We have analyzed the morphology of ferroelectric domains in very thin films
of multiferroic BiFeO3. Unlike the more common stripe domains observed in
thicker films BiFeO3 or in other ferroics, the domains tend not to be straight,
but irregular in shape, with significant domain wall roughening leading to a
fractal dimensionality. Also contrary to what is usually observed in other
ferroics, the domain size appears not to scale as the square root of the film
thickness. A model is proposed in which the observed domain size as a function
of film thickness can be directly linked to the fractal dimension of the
domains.Comment: 4 pages, 3 figure
Landau Theory of Domain Wall Magnetoelectricity
We calculate the exact analytical solution to the domain wall properties in a
multiferroic system with two order parameters that are coupled
bi-quadratically. This is then adapted to the case of a magnetoelectric
multiferroic material such as BiFeO3, with a view to examine critically whether
the domain walls can account for the enhancement of magnetization reported for
thin films fo this material, in view of the correlation between increasing
magnetization and increasing volume fraction of domain walls as films become
thinner. The present analysis can be generalized to describe a class of
magnetoelectric devices based upon domain walls rather than bulk properties.Comment: 9 pages, 4 figure
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