6,479 research outputs found
Magnetism of two-dimensional defects in Pd: stacking faults, twin boundaries and surfaces
Careful first-principles density functional calculations reveal the
importance of hexagonal versus cubic stacking of closed packed planes of Pd as
far as local magnetic properties are concerned. We find that, contrary to the
stable face centered cubic phase, which is paramagnetic, the hexagonal
close-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35
/atom. Our results show that two-dimensional defects with local hcp
stacking, like twin boundaries and stacking faults, in the otherwise fcc Pd
structure, increase the magnetic susceptibility. The (111) surface also
increases the magnetic susceptibility and it becomes ferromagnetic in
combination with an individual stacking fault or twin boundary close to it. On
the contrary, we find that the (100) surface decreases the tendency to
ferromagnetism. The results are consistent with the magnetic moment recently
observed in small Pd nanoparticles, with a large surface area and a high
concentration of two-dimensional stacking defects.Comment: 8 pages, 10 figure
An Imprint of Molecular Cloud Magnetization in the Morphology of the Dust Polarized Emission
We describe a morphological imprint of magnetization found when considering
the relative orientation of the magnetic field direction with respect to the
density structures in simulated turbulent molecular clouds. This imprint was
found using the Histogram of Relative Orientations (HRO): a new technique that
utilizes the gradient to characterize the directionality of density and column
density structures on multiple scales. We present results of the HRO analysis
in three models of molecular clouds in which the initial magnetic field
strength is varied, but an identical initial turbulent velocity field is
introduced, which subsequently decays. The HRO analysis was applied to the
simulated data cubes and mock-observations of the simulations produced by
integrating the data cube along particular lines of sight. In the 3D analysis
we describe the relative orientation of the magnetic field with
respect to the density structures, showing that: 1.The magnetic field shows a
preferential orientation parallel to most of the density structures in the
three simulated cubes. 2.The relative orientation changes from parallel to
perpendicular in regions with density over a critical density in the
highest magnetization case. 3.The change of relative orientation is largest for
the highest magnetization and decreases in lower magnetization cases. This
change in the relative orientation is also present in the projected maps. In
conjunction with simulations HROs can be used to establish a link between the
observed morphology in polarization maps and the physics included in
simulations of molecular clouds.Comment: (16 pages, 11 figures, submitted to ApJ 05MAR2013, accepted
07JUL2013
Iron oxide doped boron nitride nanotubes: structural and magnetic properties
A first-principles formalism is employed to investigate the interaction of
iron oxide (FeO) with a boron nitride (BN) nanotube. The stable structure of
the FeO-nanotube has Fe atoms binding N atoms, with bond length of roughly
2.1 \AA, and binding between O and B atoms, with bond length of 1.55 \AA.
In case of small FeO concentrations, the total magnetic moment is
(4) times the number of Fe atoms in the unit cell and it is
energetically favorable to FeO units to aggregate rather than randomly bind to
the tube. As a larger FeO concentration case, we study a BN nanotube fully
covered by a single layer of FeO. We found that such a structure has square FeO
lattice with Fe-O bond length of 2.11 \AA, similar to that of FeO bulk, and
total magnetic moment of 3.94 per Fe atom. Consistently with
experimental results, the FeO covered nanotube is a semi-half-metal which can
become a half-metal if a small change in the Fermi level is induced. Such a
structure may be important in the spintronics context.Comment: 10 pages, 3 figure
Rigid motion revisited: rigid quasilocal frames
We introduce the notion of a rigid quasilocal frame (RQF) as a geometrically
natural way to define a "system" in general relativity. An RQF is defined as a
two-parameter family of timelike worldlines comprising the worldtube boundary
of the history of a finite spatial volume, with the rigidity conditions that
the congruence of worldlines is expansion-free (constant size) and shear-free
(constant shape). This definition of a system is anticipated to yield simple,
exact geometrical insights into the problem of motion in general relativity. It
begins by answering the questions what is in motion (a rigid two-dimensional
system boundary), and what motions of this rigid boundary are possible. Nearly
a century ago Herglotz and Noether showed that a three-parameter family of
timelike worldlines in Minkowski space satisfying Born's 1909 rigidity
conditions has only three degrees of freedom instead of the six we are familiar
with from Newtonian mechanics. We argue that in fact we can implement Born's
notion of rigid motion in both flat spacetime (this paper) and arbitrary curved
spacetimes containing sources (subsequent papers) - with precisely the expected
three translational and three rotational degrees of freedom - provided the
system is defined quasilocally as the two-dimensional set of points comprising
the boundary of a finite spatial volume, rather than the three-dimensional set
of points within the volume.Comment: 10 pages (two column), 24 pages (preprint), 1 figur
Anomalous Nuclear Quantum Effects in Ice
One striking anomaly of water ice has been largely neglected and never
explained. Replacing hydrogen (H) by deuterium (H) causes ice to
expand, whereas the "normal" isotope effect is volume contraction with
increased mass. Furthermore, the anomaly increases with temperature , even
though a normal isotope shift should decrease with and vanish when is
high enough to use classical nuclear motions. In this study, we show that these
effects are very well described by {\it ab initio} density functional theory.
Our theoretical modeling explains these anomalies, and allows us to predict and
to experimentally confirm a counter effect, namely that replacement of O
by O causes a normal lattice contraction.Comment: 5 pages, 3 figure
Molecular dynamics simulations of lead clusters
Molecular dynamics simulations of nanometer-sized lead clusters have been
performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf
269/270}, 1109 (1992)). The binding energies of clusters forming crystalline
(fcc), decahedron and icosahedron structures are compared, showing that fcc
cuboctahedra are the most energetically favoured of these polyhedral model
structures. However, simulations of the freezing of liquid droplets produced a
characteristic form of ``shaved'' icosahedron, in which atoms are absent at the
edges and apexes of the polyhedron. This arrangement is energetically favoured
for 600-4000 atom clusters. Larger clusters favour crystalline structures.
Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect
fcc Wulff particle, containing a number of parallel stacking faults. The
effects of temperature on the preferred structure of crystalline clusters below
the melting point have been considered. The implications of these results for
the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other
minor changes for publicatio
- …