58,413 research outputs found
Role of internal gases and creep of Ag in controlling the critical current density of Ag-sheathed Bi2Sr2CaCu2Ox wires
High engineering critical current density JE of >500 A/mm2 at 20 T and 4.2 K
can be regularly achieved in Ag-sheathed multifilamentary Bi2Sr2CaCu2Ox
(Bi-2212) round wire when the sample length is several centimeters. However,
JE(20 T) in Bi-2212 wires of several meters length, as well as longer pieces
wound in coils, rarely exceeds 200 A/mm2. Moreover, long-length wires often
exhibit signs of Bi-2212 leakage after melt processing that are rarely found in
short, open-end samples. We studied the length dependence of JE of
state-of-the-art powder-in-tube (PIT) Bi-2212 wires and gases released by them
during melt processing using mass spectroscopy, confirming that JE degradation
with length is due to wire swelling produced by high internal gas pressures at
elevated temperatures [1,2]. We further modeled the gas transport in Bi-2212
wires and examined the wire expansion at critical stages of the melt processing
of as-drawn PIT wires and the wires that received a degassing treatment or a
cold-densification treatment before melt processing. These investigations
showed that internal gas pressure in long-length wires drives creep of the Ag
sheath during the heat treatment, causing wire to expand, lowering the density
of Bi-2212 filaments, and therefore degrading the wire JE; the creep rupture of
silver sheath naturally leads to the leakage of Bi-2212 liquid. Our work shows
that proper control of such creep is the key to preventing Bi-2212 leakage and
achieving high JE in long-length Bi-2212 conductors and coils
Gather-Excite: Exploiting Feature Context in Convolutional Neural Networks
While the use of bottom-up local operators in convolutional neural networks
(CNNs) matches well some of the statistics of natural images, it may also
prevent such models from capturing contextual long-range feature interactions.
In this work, we propose a simple, lightweight approach for better context
exploitation in CNNs. We do so by introducing a pair of operators: gather,
which efficiently aggregates feature responses from a large spatial extent, and
excite, which redistributes the pooled information to local features. The
operators are cheap, both in terms of number of added parameters and
computational complexity, and can be integrated directly in existing
architectures to improve their performance. Experiments on several datasets
show that gather-excite can bring benefits comparable to increasing the depth
of a CNN at a fraction of the cost. For example, we find ResNet-50 with
gather-excite operators is able to outperform its 101-layer counterpart on
ImageNet with no additional learnable parameters. We also propose a parametric
gather-excite operator pair which yields further performance gains, relate it
to the recently-introduced Squeeze-and-Excitation Networks, and analyse the
effects of these changes to the CNN feature activation statistics.Comment: NeurIPS 201
Anomalous high energy dispersion in photoemission spectra from insulating cuprates
Angle resolved photoelectron spectroscopic measurements have been performed
on an insulating cuprate Ca_2CuO_2Cl_2. High resolution data taken along the
\Gamma to (pi,pi) cut show an additional dispersive feature that merges with
the known dispersion of the lowest binding energy feature, which follows the
usual strongly renormalized dispersion of ~0.35 eV. This higher energy part
reveals a dispersion that is very close to the unrenormalized band predicted by
band theory. A transfer of spectral weight from the low energy feature to the
high energy feature is observed as the \Gamma point is approached. By comparing
with theoretical calculations the high energy feature observed here
demonstrates that the incoherent portion of the spectral function has
significant structure in momentum space due to the presence of various energy
scales.Comment: 5 pages, 3 figure
Algorithmic statistics: forty years later
Algorithmic statistics has two different (and almost orthogonal) motivations.
From the philosophical point of view, it tries to formalize how the statistics
works and why some statistical models are better than others. After this notion
of a "good model" is introduced, a natural question arises: it is possible that
for some piece of data there is no good model? If yes, how often these bad
("non-stochastic") data appear "in real life"?
Another, more technical motivation comes from algorithmic information theory.
In this theory a notion of complexity of a finite object (=amount of
information in this object) is introduced; it assigns to every object some
number, called its algorithmic complexity (or Kolmogorov complexity).
Algorithmic statistic provides a more fine-grained classification: for each
finite object some curve is defined that characterizes its behavior. It turns
out that several different definitions give (approximately) the same curve.
In this survey we try to provide an exposition of the main results in the
field (including full proofs for the most important ones), as well as some
historical comments. We assume that the reader is familiar with the main
notions of algorithmic information (Kolmogorov complexity) theory.Comment: Missing proofs adde
Anti-shielding Effect and Negative Temperature in Instantaneously Reversed Electric Fields and Left-Handed Media
The connections between the anti-shielding effect, negative absolute
temperature and superluminal light propagation in both the instantaneously
reversed electric field and the left-handed media are considered in the present
paper. The instantaneous inversion of the exterior electric field may cause the
electric dipoles into the state of negative absolute temperature and therefore
give rise to a negative effective mass term of electromagnetic field (i. e.,
the electromagnetic field propagating inside the negative-temperature medium
will acquire an imaginary rest mass), which is said to result in the potential
superluminality effect of light propagation in this anti-shielding dielectric.
In left-handed media, such phenomena may also arise.Comment: 9 pages, Late
Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy
The ferromagnetism of a thin GaMnAs layer with a perpendicular easy
anisotropy axis is investigated by means of several techniques, that yield a
consistent set of data on the magnetic properties and the domain structure of
this diluted ferromagnetic semiconductor. The magnetic layer was grown under
tensile strain on a relaxed GaInAs buffer layer using a procedure that limits
the density of threading dislocations. Magnetometry, magneto-transport and
polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality
of this layer, in particular through its high Curie temperature (130 K) and
well-defined magnetic anisotropy. We show that magnetization reversal is
initiated from a limited number of nucleation centers and develops by easy
domain wall propagation. Furthermore, MOKE microscopy allowed us to
characterize in detail the magnetic domain structure. In particular we show
that domain shape and wall motion are very sensitive to some defects, which
prevents a periodic arrangement of the domains. We ascribed these defects to
threading dislocations emerging in the magnetic layer, inherent to the growth
mode on a relaxed buffer
Screening of nuclear pairing in nuclear and neutron matter
The screening potential in the and pairing channels in
neutron and nuclear matter in different approximations is discussed. It is
found that the vertex corrections to the potential are much stronger in nuclear
matter than in neutron matter.Comment: 11 pages, 8 figures, revtex4 styl
- …