5,762 research outputs found
Magnetic properties of undoped Cu2O fine powders with magnetic impurities and/or cation vacancies
Fine powders of micron- and submicron-sized particles of undoped Cu2O
semiconductor, with three different sizes and morphologies have been
synthesized by different chemical processes. These samples include nanospheres
200 nm in diameter, octahedra of size 1 micron, and polyhedra of size 800 nm.
They exhibit a wide spectrum of magnetic properties. At low temperature, T = 5
K, the octahedron sample is diamagnetic. The nanosphere is paramagnetic. The
other two polyhedron samples synthesized in different runs by the same process
are found to show different magnetic properties. One of them exhibits weak
ferromagnetism with T_C = 455 K and saturation magnetization, M_S = 0.19 emu/g
at T = 5 K, while the other is paramagnetic. The total magnetic moment
estimated from the detected impurity concentration of Fe, Co, and Ni, is too
small to account for the observed magnetism by one to two orders of magnitude.
Calculations by the density functional theory (DFT) reveal that cation
vacancies in the Cu2O lattice are one of the possible causes of induced
magnetic moments. The results further predict that the defect-induced magnetic
moments favour a ferromagnetically coupled ground state if the local
concentration of cation vacancies, n_C, exceeds 12.5%. This offers a possible
scenario to explain the observed magnetic properties. The limitations of the
investigations in the present work, in particular in the theoretical
calculations, are discussed and possible areas for further study are suggested.Comment: 20 pages, 5 figures 2 tables, submitted to J Phys Condense Matte
An exact general remeshing scheme applied to physically conservative voxelization
We present an exact general remeshing scheme to compute analytic integrals of
polynomial functions over the intersections between convex polyhedral cells of
old and new meshes. In physics applications this allows one to ensure global
mass, momentum, and energy conservation while applying higher-order polynomial
interpolation. We elaborate on applications of our algorithm arising in the
analysis of cosmological N-body data, computer graphics, and continuum
mechanics problems.
We focus on the particular case of remeshing tetrahedral cells onto a
Cartesian grid such that the volume integral of the polynomial density function
given on the input mesh is guaranteed to equal the corresponding integral over
the output mesh. We refer to this as "physically conservative voxelization".
At the core of our method is an algorithm for intersecting two convex
polyhedra by successively clipping one against the faces of the other. This
algorithm is an implementation of the ideas presented abstractly by Sugihara
(1994), who suggests using the planar graph representations of convex polyhedra
to ensure topological consistency of the output. This makes our implementation
robust to geometric degeneracy in the input. We employ a simplicial
decomposition to calculate moment integrals up to quadratic order over the
resulting intersection domain.
We also address practical issues arising in a software implementation,
including numerical stability in geometric calculations, management of
cancellation errors, and extension to two dimensions. In a comparison to recent
work, we show substantial performance gains. We provide a C implementation
intended to be a fast, accurate, and robust tool for geometric calculations on
polyhedral mesh elements.Comment: Code implementation available at https://github.com/devonmpowell/r3
Relation between the weak itinerant magnetism in Ni compounds ( = Y, La) and their stacked crystal structures
The weak itinerant magnetic properties of Ni compounds with =
{Y, La} have been investigated using electronic band structure calculations in
the relation with their polymorphic crystal structures. These compounds
crystallizes in two structures resulting from the stacking of two and three
blocks of [Ni + 2 Ni] units for hexagonal -LaNi
(CeNi type) and rhombohedral -YNi (GdCo type)
respectively. Experimentally, -LaNi is a weak itinerant
antiferromagnet whereas -YNi is a weak itinerant ferromagnet. From
the present first principles calculation within non-spin polarized state, both
compounds present an electronic density of state with a sharp and narrow peak
centered at the Fermi level corresponding to flat bands from -Ni. This
induces a magnetic instability and both compounds are more stable in a
ferromagnetic (FM) order compared to a paramagnetic state (
-35 meV/f.u.). The magnetic moment of each of the five Ni sites varies with
their positions relative to the [Ni] and [Ni] units: they are
minimum in the [Ni] unit and maximum at the interface between two
[Ni] units. For -LaNi, an antiferromagnetic (AFM) structure
has been proposed and found with an energy comparable to that of the FM state.
This AFM structure is described by two FM unit blocks of opposite Ni spin sign
separated by a non-magnetic layer at z = 0 and . The Ni () atoms
belonging to this intermediate layer are located in the [LaNi] unit and
are at a center of symmetry of the hexagonal cell () where the
resultant molecular field is cancelled. Further non-collinear spin calculations
have been performed to determine the Ni moment orientations which are found
preferentially parallel to the axis for both FM and AFM structures.Comment: 19 pages, 7 figures, 2 table
Sub-Critical Closed String Field Theory in D Less Than 26
We construct the second quantized action for sub-critical closed string field
theory with zero cosmological constant in dimensions ,
generalizing the non-polynomial closed string field theory action proposed by
the author and the Kyoto and MIT groups for . The proof of gauge
invariance is considerably complicated by the presence of the Liouville field
and the non-polynomial nature of the action. However, we explicitly show
that the polyhedral vertex functions obey BRST invariance to all orders. By
point splitting methods, we calculate the anomaly contribution due to the
Liouville field, and show in detail that it cancels only if , in both the bosonized and unbosonized polyhedral vertex functions. We
also show explicitly that the four point function generated by this action
reproduces the shifted Shapiro-Virasoro amplitude found from matrix
models and Liouville theory in two dimensions. LATEX file.Comment: 28 pages, CCNY-HEP-93-
NaIrO3 - A pentavalent post-perovskite
Sodium iridium(V) oxide, NaIrO3, was synthesized by a high pressure solid
state method and recovered to ambient conditions. It is found to be
isostructural with CaIrO3, the much-studied structural analogue of the
high-pressure post-perovskite phase of MgSiO3. Among the oxide
post-perovskites, NaIrO3 is the first example with a pentavalent cation. The
structure consists of layers of corner- and edge-sharing IrO6 octahedra
separated by layers of NaO8 bicapped trigonal prisms. NaIrO3 shows no magnetic
ordering and resistivity measurements show non-metallic behavior. The crystal
structure, electrical and magnetic properties are discussed and compared to
known post-perovskites and pentavalent perovskite metal oxides.Comment: 22 pages, 5 figures. Submitted to Journal of Solid State Chemistr
Software for Exact Integration of Polynomials over Polyhedra
We are interested in the fast computation of the exact value of integrals of
polynomial functions over convex polyhedra. We present speed ups and extensions
of the algorithms presented in previous work. We present the new software
implementation and provide benchmark computations. The computation of integrals
of polynomials over polyhedral regions has many applications; here we
demonstrate our algorithmic tools solving a challenge from combinatorial voting
theory.Comment: Major updat
On the equivalence between the cell-based smoothed finite element method and the virtual element method
We revisit the cell-based smoothed finite element method (SFEM) for
quadrilateral elements and extend it to arbitrary polygons and polyhedrons in
2D and 3D, respectively. We highlight the similarity between the SFEM and the
virtual element method (VEM). Based on the VEM, we propose a new stabilization
approach to the SFEM when applied to arbitrary polygons and polyhedrons. The
accuracy and the convergence properties of the SFEM are studied with a few
benchmark problems in 2D and 3D linear elasticity. Later, the SFEM is combined
with the scaled boundary finite element method to problems involving
singularity within the framework of the linear elastic fracture mechanics in
2D
Inverse pressure-induced Mott transition in TiPO
TiPO shows interesting structural and magnetic properties as temperature
and pressure are varied, such as a spin-Peierls phase transition and the
development of incommensurate modulations of the lattice. Recently, high
pressure experiments for TiPO reported two new structural phases appearing
at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem.
Int. Ed. 55, 15053]. The latter was shown to include the first example of
5-fold O-coordinated P-atoms in an inorganic phosphate compound. In this work
we characterize the electronic structure and other physical properties of these
new phases by means of ab-initio calculations, and investigate the structural
transition. We find that the appearance of phases IV and V coincides with a
collapse of the Mott insulating gap and quenching of magnetism in phase III as
pressure is applied. Remarkably, our calculations show that in the high
pressure phase V, these features reappear, leading to an antiferromagnetic Mott
insulating phase, with robust local moments
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