522 research outputs found
EBSD mapping of herringbone domain structures in tetragonal piezoelectrics
Herringbone domain structures have been mapped using electron backscatter diffraction (EBSD) in two tetragonal piezoelectrics, lead zirconate titanate, [Pb(Zr,Ti)O<sub>3</sub>] and bismuth ferrite – lead titanate, [(PbTi)<sub>0.5</sub>(BiFe)<sub>0.5</sub>O<sub>3</sub>]. Analysis of the domain misorientations across the band junctions shows that the structures correspond very well to crystallographic models. High resolution mapping with a 20 nm step size allowed the crystal rotation across one of these band junctions in lead zirconate titanate to be studied in detail and allowed an improved estimation of the peak strain at the junction, of 0.56 GPa. The significance of this for crack nucleation and propagation in such materials is discussed
Island phases and charge order in two-dimensional manganites
The ferromagnetic Kondo lattice model with an antiferromagnetic interaction
between localized spins is a minimal description of the competing kinetic t and
magnetic K energy terms which generate the rich physics of manganite systems.
Motivated by the discovery in one dimension of homogeneous ``island phases'',
we consider the possibility of analogous phases in higher dimensions. We
characterize the phases present at commensurate fillings, and consider in
detail the effects of phase separation in all filling and parameter regimes. We
deduce that island and flux phases are stable for intermediate values of K/t at
the commensurate fillings n = 1/4, 1/3, 3/8, and 1/2. We discuss the connection
of these results to the charge and magnetic ordering observed in a wide variety
of manganite compounds.Comment: 13 pages, 17 figure
Analysis of the Data from Compton X-ray Polarimeters which Measure the Azimuthal and Polar Scattering Angles
X-ray polarimetry has the potential to make key-contributions to our
understanding of galactic compact objects like binary black hole systems and
neutron stars, and extragalactic objects like active galactic nuclei, blazars,
and Gamma Ray Bursts. Furthermore, several particle astrophysics topics can be
addressed including uniquely sensitive tests of Lorentz invariance. In the
energy range from 10 keV to several MeV, Compton polarimeters achieve the best
performance. In this paper we evaluate the benefit that comes from using the
azimuthal and polar angles of the Compton scattered photons in the analysis,
rather than using the azimuthal scattering angles alone. We study the case of
an ideal Compton polarimeter and show that a Maximum Likelihood analysis which
uses the two scattering angles lowers the Minimum Detectable Polarization (MDP)
by ~20% compared to a standard analysis based on the azimuthal scattering
angles alone. The accuracies with which the polarization fraction and the
polarization direction can be measured improve by a similar amount. We conclude
by discussing potential applications of Maximum Likelihood analysis methods for
various polarimeter experiments.Comment: Accepted for publication in Astroparticle Physics (14 pages, 4
figures
A Quantitative Non-radial Oscillation Model for the Subpulses in PSR B0943+10
In this paper, we analyze time series measurements of PSR B0943+10 and fit
them with a non-radial oscillation model. The model we apply was first
developed for total intensity measurements in an earlier paper, and expanded to
encompass linear polarization in a companion paper to this one. We use PSR
B0943+10 for the initial tests of our model because it has a simple geometry,
it has been exhaustively studied in the literature, and its behavior is
well-documented. As prelude to quantitative fitting, we have reanalyzed
previously published archival data of PSR B0943+10 and uncovered subtle but
significant behavior that is difficult to explain in the framework of the
drifting spark model. Our fits of a non-radial oscillation model are able to
successfully reproduce the observed behavior in this pulsar.Comment: 45 pages, 16 figures, accepted Ap
The Dam1 ring binds to the E-hook of tubulin and diffuses along the microtubule.
There has been much effort in recent years aimed at understanding the molecular mechanism by which the Dam1 kinetochore complex is able to couple microtubule depolymerization to poleward movement. Both a biased diffusion and a forced walk model have been proposed, and several key functional aspects of Dam1-microtubule binding are disputed. Here, we investigate the elements involved in tubulin-Dam1 complex interactions and directly visualize Dam1 rings on microtubules in order to infer their dynamic behavior on the microtubule lattice and its likely relevance at the kinetochore. We find that the Dam1 complex has a preference for native tubulin over tubulin that is lacking its acidic C-terminal tail. Statistical mechanical analysis of images of Dam1 rings on microtubules, applied to both the distance between rings and the tilt angle of the rings with respect to the microtubule axis, supports a diffusive ring model. We also present a cryo-EM reconstruction of the Dam1 ring, likely the relevant assembly form of the complex for energy coupling during microtubule depolymerization in budding yeast. The present studies constitute a significant step forward by linking structural and biochemical observations toward a comprehensive understanding of the Dam1 complex
Using EBSD and TEM-Kikuchi patterns to study local crystallography at the domain boundaries of lead zirconate titanate
Reliable EBSD mapping of 90° domains in a tetragonal ferroelectric perovskite has been achieved for the first time, together with reliable automated orientation determination from TEM-Kikuchi patterns. This has been used to determine misorientation angles at 90° domain boundaries and thus local <i>c</i>/<i>a</i> ratios. The sources of orientation noise/error and their effects on the misorientation angle data have been thoroughly analyzed and it is found that this gives a cosine distribution of misorientation angles about the mean with a characteristic width related to the width of the orientation noise distribution. In most cases, a good agreement is found between local <i>c</i>/<i>a</i> ratios and global measurements by X-ray diffraction, but some clear discrepancies have also been found suggesting that real local variations are present, perhaps as a consequence of compositional inhomogeneities
Well-Centered Triangulation
Meshes composed of well-centered simplices have nice orthogonal dual meshes
(the dual Voronoi diagram). This is useful for certain numerical algorithms
that prefer such primal-dual mesh pairs. We prove that well-centered meshes
also have optimality properties and relationships to Delaunay and minmax angle
triangulations. We present an iterative algorithm that seeks to transform a
given triangulation in two or three dimensions into a well-centered one by
minimizing a cost function and moving the interior vertices while keeping the
mesh connectivity and boundary vertices fixed. The cost function is a direct
result of a new characterization of well-centeredness in arbitrary dimensions
that we present. Ours is the first optimization-based heuristic for
well-centeredness, and the first one that applies in both two and three
dimensions. We show the results of applying our algorithm to small and large
two-dimensional meshes, some with a complex boundary, and obtain a
well-centered tetrahedralization of the cube. We also show numerical evidence
that our algorithm preserves gradation and that it improves the maximum and
minimum angles of acute triangulations created by the best known previous
method.Comment: Content has been added to experimental results section. Significant
edits in introduction and in summary of current and previous results. Minor
edits elsewher
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