6,736 research outputs found
Chemical vapor deposition of high T sub c superconductors
The results are reported of an investigation into the synthesis and properties of high temperature superconducting materials. A chemical vapor deposition apparatus was designed and built which is suitable for the preparation of multicomponent metal films This apparatus was used to prepare a series of high T sub c A-15 structure superconducting films in the binary system Nb-Ge. The effect on T sub c of a variety of substrate materials was investigated. An extensive series of ternary alloys were also prepared. Conditions allowing the brittle high T sub c (approximately 18 K) A-15 structure superconductor Nb3A1 to be prepared in a low T sub c but ductile form were found. Some of the ways that the ductile (bcc) form can be cold worked or machined are described. Measurements of rate of transformation of cold worked bcc material to the high T sub c A-15 structure with low temperature annealing are given. Preliminary measurements indicate that this material has attractive high field critical current densities
Students' Understanding of Direct Current Resistive Electrical Circuits
Research has shown that both high school and university students' reasoning
patterns regarding direct current resistive electric circuits often differ from
the currently accepted explanations. At present, there are no standard
diagnostic examinations in electric circuits. Two versions of a diagnostic
instrument called Determining and Interpreting Resistive Electric circuits
Concepts Tests (DIRECT) were developed, each consisting of 29 questions. The
information provided by the exam provides classroom instructors a means with
which to evaluate the progress and conceptual difficulties of their students
and their instructional methods. It can be used to evaluate curricular packages
and/or other supplemental materials for their effectiveness in overcoming
students' conceptual difficulties. The analyses indicate that students,
especially females, tend to hold multiple misconceptions, even after
instruction. During interviews, the idea that the battery is a constant source
of current was used most often in answering the questions. Students tended to
focus on current in solving the problems and to confuse terms, often assigning
the properties of current to voltage and/or resistance. Results indicated that
students do not have a clear understanding of the underlying mechanisms of
electric circuit phenomena. On the other hand, students were able to translate
easily from a "realistic" representation of a circuit to the corresponding
schematic diagram.Comment: This article has been accepted for publication in the American
Journal of Physics - Physics Education Research Supplement. No known
publication date as ye
Effects of neutral selection on the evolution of molecular species
We introduce a new model of evolution on a fitness landscape possessing a
tunable degree of neutrality. The model allows us to study the general
properties of molecular species undergoing neutral evolution. We find that a
number of phenomena seen in RNA sequence-structure maps are present also in our
general model. Examples are the occurrence of "common" structures which occupy
a fraction of the genotype space which tends to unity as the length of the
genotype increases, and the formation of percolating neutral networks which
cover the genotype space in such a way that a member of such a network can be
found within a small radius of any point in the space. We also describe a
number of new phenomena which appear to be general properties of neutrally
evolving systems. In particular, we show that the maximum fitness attained
during the adaptive walk of a population evolving on such a fitness landscape
increases with increasing degree of neutrality, and is directly related to the
fitness of the most fit percolating network.Comment: 16 pages including 4 postscript figures, typeset in LaTeX2e using the
Elsevier macro package elsart.cl
Regarding Distant Suffering : Audience engagement with representations of humanitarian disaster
Regarding Distant Suffering : Audience engagement with representations of humanitarian disaster
Writhe of center vortices and topological charge -- an explicit example
The manner in which continuum center vortices generate topological charge
density is elucidated using an explicit example. The example vortex
world-surface contains one lone self-intersection point, which contributes a
quantum 1/2 to the topological charge. On the other hand, the surface in
question is orientable and thus must carry global topological charge zero due
to general arguments. Therefore, there must be another contribution, coming
from vortex writhe. The latter is known for the lattice analogue of the example
vortex considered, where it is quite intuitive. For the vortex in the
continuum, including the limit of an infinitely thin vortex, a careful analysis
is performed and it is shown how the contribution to the topological charge
induced by writhe is distributed over the vortex surface.Comment: 33 latex pages, 10 figures incorporating 14 ps files. Furthermore,
the time evolution of the vortex line discussed in this work can be viewed as
a gif movie, available for download by following the PostScript link below --
watch for the cute feature at the self-intersection poin
Energy Density of Vortices in the Schroedinger Picture
The one-loop energy density of an infinitely thin static magnetic vortex in
SU(2) Yang-Mills theory is evaluated using the Schroedinger picture. Both the
gluonic fluctuations as well as the quarks in the vortex background are
included. The energy density of the magnetic vortex is discussed as a function
of the magnetic flux. The center vortices correspond to local minima in the
effective potential. These minima are degenerated with the perturbative vacuum
if the fermions are ignored. Inclusion of fermions lifts this degeneracy,
raising the vortex energy above the energy of the perturbative vacuum.Comment: 25 pages, 2 figure
Lattice QCD study of the Boer-Mulders effect in a pion
The three-dimensional momenta of quarks inside a hadron are encoded in
transverse momentum-dependent parton distribution functions (TMDs). This work
presents an exploratory lattice QCD study of a TMD observable in the pion
describing the Boer-Mulders effect, which is related to polarized quark
transverse momentum in an unpolarized hadron. Particular emphasis is placed on
the behavior as a function of a Collins-Soper evolution parameter quantifying
the relative rapidity of the struck quark and the initial hadron, e.g., in a
semi-inclusive deep inelastic scattering (SIDIS) process. The lattice
calculation, performed at the pion mass m_pi = 518 MeV, utilizes a definition
of TMDs via hadronic matrix elements of a quark bilocal operator with a
staple-shaped gauge connection; in this context, the evolution parameter is
related to the staple direction. By parametrizing the aforementioned matrix
elements in terms of invariant amplitudes, the problem can be cast in a Lorentz
frame suited for the lattice calculation. In contrast to an earlier nucleon
study, due to the lower mass of the pion, the calculated data enable
quantitative statements about the physically interesting limit of large
relative rapidity. In passing, the similarity between the Boer-Mulders effects
extracted in the pion and the nucleon is noted.Comment: 16 pages, 9 figures, 3 table
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