404 research outputs found
Maghemite-like regions at crossing of two antiphase boundaries in doped BiFeO3
We report the observation of a novel structure at the point where two antiphase boundaries cross in a doped bismuth ferrite of composition (Bi0.85Nd0.15)(Fe0.9Ti0.1)O0.3. The structure was investigated using a combination of high angle annular dark field imaging and electron energy loss spectroscopy spectrum imaging in the scanning transmission electron microscope. A three-dimensional model was constructed by combining the position and chemistry data with previous results and assuming octahedral coordination of all Fe and Ti atoms. The resulting structure shows some novel L shaped arrangements of iron columns, which are coordinated in a similar manner to FeO6 octahedra in maghemite. It is suggested that this may lead to local ferromagnetic orderings similar to those in maghemite
Novel nanorod precipitate formation in neodymium and titanium codoped bismuth ferrite
The discovery of unusual nanorod precipitates in bismuth ferrite doped with Nd and Ti is reported. The atomic structure and chemistry of the nanorods are determined using a combination of high angle annular dark field imaging, electron energy loss spectroscopy, and density functional calculations. It is found that the structure of the BiFeO3 matrix is strongly modified adjacent to the precipitates; the readiness of BiFeO3 to adopt different structural allotropes in turn explains why such a large axial ratio, uncommon in precipitates, is stabilized. In addition, a correlation is found between the alignment of the rods and the orientation of ferroelastic domains in the matrix, which is consistent with the system's attempt to minimize its internal strain. Density functional calculations indicate a finite density of electronic states at the Fermi energy within the rods, suggesting enhanced electrical conductivity along the rod axes, and motivating future investigations of nanorod functionalities
New examples of Willmore submanifolds in the unit sphere via isoparametric functions,II
This paper is a continuation of a paper with the same title of the last two
authors. In the first part of the present paper, we give a unified geometric
proof that both focal submanifolds of every isoparametric hypersurface in
spheres with four distinct principal curvatures are Willmore. In the second
part, we completely determine which focal submanifolds are Einstein except one
case.Comment: 19 pages,to appear in Annals of Global Analysis and Geometr
Techniques of replica symmetry breaking and the storage problem of the McCulloch-Pitts neuron
In this article the framework for Parisi's spontaneous replica symmetry
breaking is reviewed, and subsequently applied to the example of the
statistical mechanical description of the storage properties of a
McCulloch-Pitts neuron. The technical details are reviewed extensively, with
regard to the wide range of systems where the method may be applied. Parisi's
partial differential equation and related differential equations are discussed,
and a Green function technique introduced for the calculation of replica
averages, the key to determining the averages of physical quantities. The
ensuing graph rules involve only tree graphs, as appropriate for a
mean-field-like model. The lowest order Ward-Takahashi identity is recovered
analytically and is shown to lead to the Goldstone modes in continuous replica
symmetry breaking phases. The need for a replica symmetry breaking theory in
the storage problem of the neuron has arisen due to the thermodynamical
instability of formerly given solutions. Variational forms for the neuron's
free energy are derived in terms of the order parameter function x(q), for
different prior distribution of synapses. Analytically in the high temperature
limit and numerically in generic cases various phases are identified, among
them one similar to the Parisi phase in the Sherrington-Kirkpatrick model.
Extensive quantities like the error per pattern change slightly with respect to
the known unstable solutions, but there is a significant difference in the
distribution of non-extensive quantities like the synaptic overlaps and the
pattern storage stability parameter. A simulation result is also reviewed and
compared to the prediction of the theory.Comment: 103 Latex pages (with REVTeX 3.0), including 15 figures (ps, epsi,
eepic), accepted for Physics Report
Segregation of Yttrium at the Mg/MgO interface in an Mg-0.5Y Alloy
Interfacial segregation of selected elements can be exploited to manipulate the potency of solid substrates for heterogeneous nucleation, thus controlling the solidification process. As the native inclusions in Mg alloys, MgO acts as the nucleating substrate, but it has rarely been studied in terms of its interactions with alloying elements. In this work, investigations of yttrium (Y) segregation at interfaces between native MgO particles and Mg in an Mg-0.5Y alloy were carried out by state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM) and associated spectroscopy. Experimental results show that native MgO particles in Mg-0.5Y possess two typical morphologies: truncated octahedron primarily faceted by {111}MgO and minorly by {100}MgO, and cubic shape with unique {100}MgO facets. Y atoms are found to segregate at both Mg/{111}MgO and Mg/{100}MgO interfaces, leading to the formation of two different 2-dimensional compounds (2DCs). The 2DC at the Mg/{111}MgO interface is identified as two atomic layers of a face-centered cubic Y2O3 phase in terms of crystal structure and chemistry, whilst it is an Mg(Y)-O monolayer at the Mg/{100}MgO interface, coherently matching with the terminating {100}MgO plane. Discussion is focused on the mechanisms underlying the formation of the 2DCs, their effects on the nucleation potency of MgO particles, and grain refinement. This work sheds light on how heterogeneous nucleation can be manipulated by altering the nucleation potency of a substrate through deliberately promoting elemental segregation of carefully chosen element(s)
Nucleon-nucleon momentum correlation function for light nuclei
Nucleon-nucleon momentum correlation function have been presented for nuclear
reactions with neutron-rich or proton-rich projectiles using a nuclear
transport theory, namely Isospin-Dependent Quantum Molecular Dynamics model.
The relationship between the binding energy of projectiles and the strength of
proton-neutron correlation function at small relative momentum has been
explored, while proton-proton correlation function shows its sensitivity to the
proton density distribution. Those results show that nucleon-nucleon
correlation function is useful to reflect some features of the neutron- or
proton-halo nuclei and therefore provide a potential tool for the studies of
radioactive beam physics.Comment: Talk given at the 18th International IUPAP Conference on Few-Body
Problems in Physics (FB18), Santos, Brasil, August 21-26, 2006. To appear in
Nucl. Phys.
Scaling of anisotropy flows in intermediate energy heavy ion collisions
Anisotropic flows (, and ) of light nuclear clusters are
studied by a nucleonic transport model in intermediate energy heavy ion
collisions. The number-of-nucleon scalings of the directed flow () and
elliptic flow () are demonstrated for light nuclear clusters. Moreover,
the ratios of of nuclear clusters show a constant value of 1/2
regardless of the transverse momentum. The above phenomena can be understood by
the coalescence mechanism in nucleonic level and are worthy to be explored in
experiments.Comment: Invited talk at "IX International Conference on Nucleus-Nucleus
Collisions", Rio de Janeiro, Aug 28- Sept 1, 2006; to appear on the
proceeding issue in Nuclear Physics
Search for the Rare Decays J/Psi --> Ds- e+ nu_e, J/Psi --> D- e+ nu_e, and J/Psi --> D0bar e+ e-
We report on a search for the decays J/Psi --> Ds- e+ nu_e + c.c., J/Psi -->
D- e+ nu_e + c.c., and J/Psi --> D0bar e+ e- + c.c. in a sample of 5.8 * 10^7
J/Psi events collected with the BESII detector at the BEPC. No excess of signal
above background is observed, and 90% confidence level upper limits on the
branching fractions are set: B(J/Psi --> Ds- e+ nu_e + c.c.)<4.8*10^-5, B(J/Psi
--> D- e+ nu_e + c.c.) D0bar e+ e- + c.c.)<1.1*10^-5Comment: 10 pages, 4 figure
Bimetallic copper palladium nanorods: plasmonic properties and palladium content effects
Cu is an inexpensive alternative plasmonic metal with optical behaviour comparable to Au but with much poorer environmental stability. Alloying with a more stable metal can improve stability and add functionality, with potential effects on the plasmonic properties. Here we investigate the plasmonic behaviour of Cu nanorods and Cu–CuPd nanorods containing up to 46 mass percent Pd. Monochromated scanning transmission electron microscopy electron energy-loss spectroscopy first reveals the strong length dependence of multiple plasmonic modes in Cu nanorods, where the plasmon peaks redshift and narrow with increasing length. Next, we observe an increased damping (and increased linewidth) with increasing Pd content, accompanied by minimal frequency shift. These results are corroborated by and expanded upon with numerical simulations using the electron-driven discrete dipole approximation. This study indicates that adding Pd to nanostructures of Cu is a promising method to expand the scope of their plasmonic applications
Direct Measurements of the Branching Fractions for and and Determinations of the Form Factors and
The absolute branching fractions for the decays and
are determined using singly
tagged sample from the data collected around 3.773 GeV with the
BES-II detector at the BEPC. In the system recoiling against the singly tagged
meson, events for and events for decays are observed. Those yield
the absolute branching fractions to be and . The
vector form factors are determined to be
and . The ratio of the two form
factors is measured to be .Comment: 6 pages, 5 figure
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