391 research outputs found
Optical studies of ferroelectric and ferroelastic domain walls
Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the domains, such as enhanced conductivity in insulators, or polar properties in non-polar materials. In this review we show that optical techniques, in spite of the diffraction limit, also provide key insights into the structure and physical properties of ferroelectric and ferroelastic domain walls. We give an overview of the uses, specificities and limits of these techniques, and emphasize the properties of the domain walls that they can probe. We then highlight some open questions of the physics of domain walls that could benefit from their use
Elastic anomalies associated with domain switching in BaTiO3 single crystals under in situ electrical cycling
The elastic response of BaTiO3 single crystals during electric field cycling at room temperature has been studied using in situ Resonant Ultrasound Spectroscopy (RUS), which allows monitoring of both the elastic and anelastic changes caused by ferroelectric polarization switching. We find that the first ferroelectric switching of a virgin single crystal is dominated by ferroelastic 90° switching. In subsequent ferroelectric switching, ferroelastic switching is reduced by domain pinning and by the predominance of 180° ferroelectric domains, as confirmed by polarized light microscopy. RUS under in situ electric field therefore demonstrates to be an effective technique for the investigation of electromechanical coupling in ferroelectrics
The Predicted Properties of Helium-Enriched Globular Cluster Progenitors at High Redshift
Globular cluster progenitors may have been detected by \textit{HST}, and are
predicted to be observable with \textit{JWST} and ground-based extremely-large
telescopes with adaptive optics. This has the potential to elucidate the issue
of globular cluster formation and the origins of significantly helium-enriched
subpopulations, a problem in Galactic astronomy with no satisfactory
theoretical solution. Given this context, we use model stellar tracks and
isochrones to investigate the predicted observational properties of
helium-enriched stellar populations in globular cluster progenitors. We find
that, relative to helium-normal populations, helium-enriched
() stellar populations similar to those inferred in the most
massive globular clusters, are expected, modulo some rapid fluctuations in the
first 30 Myr, to be brighter and redder in the rest frame. At fixed age,
stellar mass, and metallicity, a helium-enriched population is predicted to
converge to being 0.40 mag brighter at ,
and to be 0.30 mag redder in the \textit{JWST}-NIRCam colour ,
and to actually be fainter for . Separately,
we find that the time-integrated shift in ionizing radiation is a negligible
, though we show that the Lyman- escape fraction could end up
higher for helium-enriched stars.Comment: Accepted for publication in MNRAS on 13 May, 2020 Replaced to update
some bibliographic informatio
The origin of the split red clump in the Galactic bulge of the Milky Way
Near the minor axis of the Galactic bulge, at latitudes b < -5 degrees, the
red giant clump stars are split into two components along the line of sight. We
investigate this split using the three fields from the ARGOS survey that lie on
the minor axis at (l,b) = (0,-5), (0,-7.5), (0,-10) degrees. The separation is
evident for stars with [Fe/H] > -0.5 in the two higher-latitude fields, but not
in the field at b = -5 degrees. Stars with [Fe/H] < -0.5 do not show the split.
We compare the spatial distribution and kinematics of the clump stars with
predictions from an evolutionary N-body model of a bulge that grew from a disk
via bar-related instabilities. The density distribution of the peanut-shaped
model is depressed near its minor axis. This produces a bimodal distribution of
stars along the line of sight through the bulge near its minor axis, very much
as seen in our observations. The observed and modelled kinematics of the two
groups of stars are also similar. We conclude that the split red clump of the
bulge is probably a generic feature of boxy/peanut bulges that grew from disks,
and that the disk from which the bulge grew had relatively few stars with
[Fe/H] < -0.5Comment: 12 pages, 9 figures, accepted for publication in Ap
Selective area growth of a- and c-plane GaN nanocolumns by molecular beam epitaxy using colloidal nanolithography
Selective area growth of a-plane GaN nanocolumns by molecular beam epitaxy was performed for the first time on a-plane GaN templates. Ti masks with 150 nm diameter nanoholes were fabricated by colloidal lithography, an easy, fast and cheap process capable to handle large areas. Even though colloidal lithography does not provide a perfect geometrical arrangement like e-beam lithography, it produces a very homogeneous mask in terms of nanohole diameter and density, and is used here for the first time for the selective area growth of GaN. Selective area growth of a-plane GaN nanocolumns is compared, in terms of anisotropic lateral and vertical growth rates, with GaN nanocolumns grown selectively on the c-plan
The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge
We present the first results of the EMBLA survey (Extremely Metal-poor BuLge
stars with AAOmega), aimed at finding metal-poor stars in the Milky Way bulge,
where the oldest stars should now preferentially reside. EMBLA utilises
SkyMapper photometry to pre-select metal-poor candidates, which are
subsequently confirmed using AAOmega spectroscopy. We describe the discovery
and analysis of four bulge giants with -2.72<=[Fe/H]<=-2.48, the lowest
metallicity bulge stars studied with high-resolution spectroscopy to date.
Using FLAMES/UVES spectra through the Gaia-ESO Survey we have derived
abundances of twelve elements. Given the uncertainties, we find a chemical
similarity between these bulge stars and halo stars of the same metallicity,
although the abundance scatter may be larger, with some of the stars showing
unusual [{\alpha}/Fe] ratios.Comment: 7 pages, 5 figures. Accepted for publication by MNRA
Filtering of Defects in Semipolar (11â22) GaN Using 2-Steps Lateral Epitaxial Overgrowth
Good-quality (11â22) semipolar GaN sample was obtained using epitaxial lateral overgrowth. The growth conditions were chosen to enhance the growth rate along the [0001] inclined direction. Thus, the coalescence boundaries stop the propagation of basal stacking faults. The faults filtering and the improvement of the crystalline quality were attested by transmission electron microscopy and low temperature photoluminescence. The temperature dependence of the luminescence polarization under normal incidence was also studied
Ordered gan/ingan nanorods arrays grown by molecular beam epitaxy for phosphor-free white light emission
The basics of the self-assembled growth of GaN nanorods on Si(111) are reviewed. Morphology differences and optical properties are compared to those of GaN layers grown directly on Si(111). The effects of the growth temperature on the In incorporation in self-assembled InGaN nanorods grown on Si(111) is described. In addition, the inclusion of InGaN quantum disk structures into selfassembled GaN nanorods show clear confinement effects as a function of the quantum disk thickness. In order to overcome the properties dispersion and the intrinsic inhomogeneous nature of the self-assembled growth, the selective area growth of GaN nanorods on both, c-plane and a-plane GaN on sapphire templates, is addressed, with special emphasis on optical quality and morphology differences. The analysis of the optical emission from a single InGaN quantum disk is shown for both polar and non-polar nanorod orientation
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