12,173 research outputs found
Density matrix renormalization group study of conjugated polymers with transverse pi-conjugation
We report accurate numerical studies of excited state orderings in long
hypothetical pi-conjugated oligomers in which the hydrogen atoms of
trans-polyacetylene are replaced with conjugated sidegroups, within modified
Hubbard models. There exists a range of the bare Coulomb repulsion for which
the excited state ordering is conducive to photoluminescence in the substituted
systems, even as this ordering is opposite in the unsubstituted polyenes of the
same lengths. Our work provides motivation to study real pi-conjugated polymers
with transverse conjugation and small optical gaps.Comment: 5 pages, 4 fig
Wear and Friction Modeling on Lifeboat Launch Systems
The RNLI provides search and rescue cover along the UK and RoI coast using a variety of lifeboats
and launch techniques. In locations where there is no natural harbour it is necessary to use a slipway
to launch the lifeboat into the sea. Lifeboat slipway stations consist of an initial section where the
boat is held on rollers followed by an inclined keelway lined with low friction composite materials,
the lifeboat is released from the top of the slipway and proceeds under its own weight into the water.
The lifeboat is later recovered using a winch line. It is common to manually apply grease to the
composite slipway lining before each launch and recovery in order to ensure sufficiently low friction
for successful operation. With the introduction of the Tamar class lifeboat it is necessary to upgrade
existing boathouses and standardise slipway operational procedures to ensure consistent operation.
The higher contact pressures associated with the new lifeboat have led to issues of high friction and
wear on the composite slipway linings and the manual application of grease to reduce friction is to be
restricted due to environmental impact and cost factors. This paper presents a multidisciplinary
approach to modelling slipway panel wear and friction using tribometer testing in conjunction with
finite element analysis and slipway condition surveys to incorporate common real-world effects such
as panel misalignments. Finally, it is shown that a freshwater lubrication system is effective,
reducing cost and environmental impacts while maintaining good friction and wear performance
Maximum-Likelihood Comparisons of Tully-Fisher and Redshift Data: Constraints on Omega and Biasing
We compare Tully-Fisher (TF) data for 838 galaxies within cz=3000 km/sec from
the Mark III catalog to the peculiar velocity and density fields predicted from
the 1.2 Jy IRAS redshift survey. Our goal is to test the relation between the
galaxy density and velocity fields predicted by gravitational instability
theory and linear biasing, and thereby to estimate where is the linear bias parameter for IRAS galaxies.
Adopting the IRAS velocity and density fields as a prior model, we maximize the
likelihood of the raw TF observables, taking into account the full range of
selection effects and properly treating triple-valued zones in the
redshift-distance relation. Extensive tests with realistic simulated galaxy
catalogs demonstrate that the method produces unbiased estimates of
and its error. When we apply the method to the real data, we model the presence
of a small but significant velocity quadrupole residual (~3.3% of Hubble flow),
which we argue is due to density fluctuations incompletely sampled by IRAS. The
method then yields a maximum likelihood estimate
(1-sigma error). We discuss the constraints on and biasing that follow
if we assume a COBE-normalized CDM power spectrum. Our model also yields the
1-D noise noise in the velocity field, including IRAS prediction errors, which
we find to be be 125 +/- 20 km/sec.Comment: 53 pages, 20 encapsulated figures, two tables. Submitted to the
Astrophysical Journal. Also available at http://astro.stanford.edu/jeff
Electron-Electron Interactions on the Edge States of Graphene: A Many Body Configuration Interaction Study
We have studied zigzag and armchair graphene nano ribbons (GNRs), described
by the Hubbard Hamiltonian using quantum many body configuration interaction
methods. Due to finite termination, we find that the bipartite nature of the
graphene lattice gets destroyed at the edges making the ground state of the
zigzag GNRs a high spin state, whereas the ground state of the armchair GNRs
remains a singlet. Our calculations of charge and spin densities suggest that,
although the electron density prefers to accumulate on the edges, instead of
spin polarization, the up and down spins prefer to mix throughout the GNR
lattice. While the many body charge gap results in insulating behavior for both
kinds of GNRs, the conduction upon application of electric field is still
possible through the edge channels because of their high electron density.
Analysis of optical states suggest differences in quantum efficiency of
luminescence for zigzag and armchair GNRs, which can be probed by simple
experiments.Comment: 5 pages, 4 figure
Inelastic collisions of ultra-cold heteronuclear molecules in an optical trap
Ultra-cold RbCs molecules in high-lying vibrational levels of the
a ground electronic state are confined in an optical trap.
Inelastic collision rates of these molecules with both Rb and Cs atoms are
determined for individual vibrational levels, across an order of magnitude of
binding energies. A simple model for the collision process is shown to
accurately reproduce the observed scattering rates
The Starburst in the Central Kiloparsec of Markarian 231
We present VLBA observations at 0.33 and 0.61 GHz, and VLA observations
between 5 and 22 GHz, of subkiloparsec scale radio emission from Mrk 231. In
addition to jet components clearly associated with the AGN, we also find a
smooth extended component of size 100 - 1000 pc most probably related to the
purported massive star forming disk in Mrk 231. The diffuse radio emission from
the disk is found to have a steep spectrum at high frequencies, characteristic
of optically thin synchrotron emission. The required relativistic particle
density in the disk can be produced by a star formation rate of 220 Msolar/yr
in the central kiloparsec. At low frequencies the disk is absorbed, most likely
by ionized gas with an emission measure of 8 x 10^5 pc cm-6. We have also
identified 4 candidate radio supernovae that, if confirmed, represent direct
evidence for ongoing star formation in the central kiloparsec.Comment: in press at ApJ for v. 519 July 1999, 14 page LaTeX document includes
6 postscript figure
Mathematik Didaktik (teaching-learning mathematics) : an overview of the development of a web-based European module
This paper outlines the development of the chapter (module) Mathematik Didaktik as a part of the electronic ‘text book’ Didaktik/Fachdidaktik developed by TNTEE Subnetwork E. The overall approach to the development of the module is based on a model of teaching-learning as an ‘integrative transformative science’ that pays due attention to the general aims of society as well as curricula, content and learning situations. As part of this perspective, teacher competence is broadly conceptualized in terms of "professional action structures" in contrast with the narrow emphasis on technical competence and on mechanistic conceptions of a ‘technology of teaching’ that currently prevail in some parts of Europe. The teaching-learning approach is based on problem-oriented, research-oriented and co-operative learning processes. Underpinning the development are particular ideas about the nature of mathematics itself. In particular the starting point for the development is around ‘big ideas’ in mathematics – in contrast to the fragmentation that is evident in the thinking of some policy makers at this time
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
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