303,069 research outputs found
Melting of regular and decoupled vortex lattices in BSCCO crystals
The angular dependence of the first-order phase transition (FOT) in the
vortex lattice in BiSrCaCuO crystals was investigated
by a low frequency AC shielding technique (with the AC field ), in
which the static-field component parallel to - () was varied with
the in-plane field held constant. The linear decrease of the
FOT field with increasing ends at a
temperature--dependent critical value of . A new transition,
marked by the abrupt drop of the -plane shielding current, appears at this
point. We draw a new phase diagram with and field
components as coordinates; this features at least two distinct regions in the
vortex solid phase, that are determined by the different interplay between the
pancake vortex-- and Josephson vortex lattice.Comment: 2 pages, 2 figures Paper submitted to the conference proceedings of
M2S-2000 Houston, T
On plane gravitational waves in real connection variables
We investigate using plane fronted gravitational wave space-times as model
systems to study loop quantization techniques and dispersion relations. In this
classical analysis, we start with planar symmetric space-times in the real
connection formulation. We reduce via Dirac constraint analysis to a final form
with one canonical pair and one constraint, equivalent to the metric and
Einstein equations of plane-fronted with parallel rays waves. Due to the
symmetries and use of special coordinates general covariance is broken.
However, this allows us to simply express the constraints of the consistent
system. A recursive construction of Dirac brackets results in non-local
brackets, analogous to those of self-dual fields, for the triad variables
chosen in this approach.Comment: v2: Matches published version, up to minor stylistic change
Signed shape tilings of squares
Let T be a tile in the Cartesian plane made up of finitely many rectangles
whose corners have rational coordinates and whose sides are parallel to the
coordinate axes. This paper gives necessary and sufficient conditions for a
square to be tilable by finitely many \Q-weighted tiles with the same shape as
T, and necessary and sufficient conditions for a square to be tilable by
finitely many \Z-weighted tiles with the same shape as T. The main tool we use
is a variant of F. W. Barnes's algebraic theory of brick packing, which
converts tiling problems into problems in commutative algebra.Comment: LaTeX, 14 pages, to appear in Discrete Mathematics. This version
differs from the original only cosmeticall
A 3D radiative transfer framework: IV. spherical & cylindrical coordinate systems
We extend our framework for 3D radiative transfer calculations with a
non-local operator splitting methods along (full) characteristics to spherical
and cylindrical coordinate systems. These coordinate systems are better suited
to a number of physical problems than Cartesian coordinates. The scattering
problem for line transfer is solved via means of an operator splitting (OS)
technique. The formal solution is based on a full characteristics method. The
approximate operator is constructed considering nearest neighbors
exactly. The code is parallelized over both wavelength and solid angle using
the MPI library. We present the results of several test cases with different
values of the thermalization parameter for the different coordinate systems.
The results are directly compared to 1D plane parallel tests. The 3D results
agree very well with the well-tested 1D calculations.Comment: A&A, in pres
Combinatorial Properties and Recognition of Unit Square Visibility Graphs
Unit square (grid) visibility graphs (USV and USGV, resp.) are described by axis-parallel visibility between unit squares placed (on integer grid coordinates) in the plane. We investigate combinatorial properties of these graph classes and the hardness of variants of the recognition problem, i.e., the problem of representing USGV with fixed visibilities within small area and, for USV, the general recognition problem
Alignment of the Angular Momentum Vectors of Planetary Nebulae in the Galactic Bulge
We use high-resolution H {\alpha} images of 130 planetary nebulae (PNe) to
investigate whether there is a preferred orientation for PNe within the
Galactic Bulge. The orientations of the full sample have an uniform
distribution. However, at a significance level of 0.01, there is evidence for a
non-uniform distribution for those planetary nebulae with evident bipolar
morphology. If we assume that the bipolar PNe have an unimodal distribution of
the polar axis in Galactic coordinates, the mean Galactic position angle is
consistent with 90{\deg}, i.e. along the Galactic plane, and the significance
level is better than 0.001 (the equivalent of a 3.7{\sigma} significance level
for a Gaussian distribution).
The shapes of PNe are related to angular momentum of the original star or
stellar system, where the long axis of the nebula measures the angular momentum
vector. In old, low-mass stars, the angular momentum is largely in binary
orbital motion. Consequently, the alignment of bipolar nebulae that we have
found indicates that the orbital planes of the binary systems are oriented
perpendicular to the Galactic plane. We propose that strong magnetic fields
aligned along the Galactic plane acted during the original star formation
process to slow the contraction of the star forming cloud in the direction
perpendicular to the plane. This would have produced a propensity for wider
binaries with higher angular momentum with orbital axes parallel to the
Galactic plane. Our findings provide the first indication of a strong,
organized magnetic field along the Galactic plane that impacted on the angular
momentum vectors of the resulting stellar population.Comment: There are two effective parts. The main paper consists of the first
17 pages and includes 8 figures and 7 tables. The remaining 10 pages will be
published as an online supplement that is made up of 4 multi-part figures.
Accepted for publication in MNRAS Main Journa
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