1,337 research outputs found
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
Summation of Series Defined by Counting Blocks of Digits
We discuss the summation of certain series defined by counting blocks of
digits in the -ary expansion of an integer. For example, if denotes
the sum of the base-2 digits of , we show that . We recover this previous
result of Sondow in math.NT/0508042 and provide several generalizations.Comment: 12 pages, Introduction expanded, references added, accepted by J.
Number Theor
Strain gradient induced polarization in SrTiO3 single crystals
Piezoelectricity is inherent only in noncentrosymmetric materials, but a
piezoelectric response can also be obtained in centrosymmetric crystals if
subjected to inhomogeneous deformation. This phenomenon, known as
flexoelectricity, affects the functional properties of insulators, particularly
thin films of high permittivity materials. We have measured
strain-gradient-induced polarization in single crystals of paraelectric
SrTiO as a function of temperature and orientation down to and below the
105 K phase transition. Estimates were obtained for all the components of the
flexoelectric tensor, and calculations based on these indicate that local
polarization around defects in SrTiO may exceed the largest ferroelectric
polarizations. A sign reversal of the flexoelectric response detected below the
phase transition suggests that the ferroelastic domain walls of SrTiO may
be polar.Comment: 4 pages, 3 figures, 1 tabl
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
The brightest pure-H ultracool white dwarf
We report the identification of LSR J0745+2627 in the United Kingdom InfraRed
Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool
white dwarf with kinematics and age compatible with the thick-disk/halo
population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high
reduced proper motion value in the J band (H_J=21.87). We show how the
infrared-reduced proper motion diagram is useful for selecting a sample of cool
white dwarfs with low contamination. LSR J0745+2627 is also detected in the
Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer
(WISE). We have spectroscopically confirmed this object as a cool white dwarf
using X-Shooter on the Very Large Telescope. A detailed analysis of its
spectral energy distribution reveals that its atmosphere is compatible with a
pure-H composition model with an effective temperature of 3880+-90 K. This
object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever
identified. We have constrained the distance (24-45 pc), space velocities and
age considering different surface gravities. The results obtained suggest that
LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the
closest ultracool white dwarfs.Comment: 5 pages, 7 figures, accepted for publication in A&A Letter
Diversification into novel habitats in the Africa clade of Dioscorea (Dioscoreaceae): erect habit and elephant's foot tubers
Background: Dioscorea is a widely distributed and highly diversified genus in tropical regions where it is represented by ten main clades, one of which diversified exclusively in Africa. In southern Africa it is characterised by a distinct group of species with a pachycaul or "elephant''s foot" structure that is partially to fully exposed above the substrate. In contrast to African representatives of the genus from other clades, occurring mainly in forest or woodland, the pachycaul taxa and their southern African relatives occur in diverse habitats ranging from woodland to open vegetation. Here we investigate patterns of diversification in the African clade, time of transition from forest to more open habitat, and morphological traits associated with each habitat and evaluate if such transitions have led to modification of reproductive organs and mode of dispersal. Results: The Africa clade originated in the Oligocene and comprises four subclades. The Dioscorea buchananii subclade (southeastern tropical Africa and South Africa) is sister to the East African subclade, which is respectively sister to the recently evolved sister South African (e. g., Cape and Pachycaul) subclades. The Cape and Pachycaul subclades diversified in the east of the Cape Peninsula in the mid Miocene, in an area with complex geomorphology and climate, where the fynbos, thicket, succulent karoo and forest biomes meet. Conclusions: Diversification out of forest is associated with major shifts in morphology of the perennial tuber (specifically an increase in size and orientation which presumably led them to become pachycaul) and rotation of stem (from twining to non-twining). The iconic elephant''s foot morphology, observed in grasslands and thicket biomes, where its corky bark may offer protection against fire and herbivory, evolved since mid Miocene. A shift in pollination trait is observed within the forest, but entry into open habitat does not show association with reproductive morphology, except in the seed wing, which has switched from winged all round the seed margin to just at the base or at the apex of it, or has been even replaced by an elaiosome
Model Atmospheres for Irradiated Stars in pre-Cataclysmic Variables
Model atmospheres have been computed for M dwarfs that are strongly
irradiated by nearby hot companions. A variety of primary and secondary
spectral types are explored in addition to models specific to four known
systems: GD 245, NN Ser, AA Dor, and UU Sge. This work demonstrates that a
dramatic temperature inversion is possible on at least one hemisphere of an
irradiated M dwarf and the emergent spectrum will be significantly different
from an isolated M dwarf or a black body flux distribution. For the first time,
synthetic spectra suitable for direct comparison to high-resolution
observations of irradiated M dwarfs in non-mass transferring post-common
envelope binaries are presented. The effects of departures from local
thermodynamic equilibrium on the Balmer line profiles are also discussed.Comment: Accepted for publication in ApJ; 12 pages, 10 figure
Quaterionic Construction of the W(F_4) Polytopes with Their Dual Polytopes and Branching under the Subgroups B(B_4) and W(B_3)*W(A_1)
4-dimensional polytopes and their dual polytopes have been
constructed as the orbits of the Coxeter-Weyl group where the group
elements and the vertices of the polytopes are represented by quaternions.
Branchings of an arbitrary \textbf{} orbit under the Coxeter groups
and have been presented. The role of
group theoretical technique and the use of quaternions have been emphasizedComment: 26 pages, 10 figure
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