397,333 research outputs found
Seniority in quantum many-body systems. I. Identical particles in a single shell
A discussion of the seniority quantum number in many-body systems is
presented. The analysis is carried out for bosons and fermions simultaneously
but is restricted to identical particles occupying a single shell. The emphasis
of the paper is on the possibility of {\em partial} conservation of seniority
which turns out to be a peculiar property of spin-9/2 fermions but prevalent in
systems of interacting bosons of any spin. Partial conservation of seniority is
at the basis of the existence of seniority isomers, frequently observed in
semi-magic nuclei, and also gives rise to peculiar selection rules in
one-nucleon transfer reactions.Comment: 41 pages, 7 figures, 1 tables, submitted to Annals of Physic
Intramolecular Imino Diels-Alder Reaction: Progress toward the Synthesis of Uncialamycin
We herein described an intramolecular imino Diels-Alder reaction promoted with BF3.OEt2/DDQ affording substituted quinolines. Using this procedure, we prepared the chiral quitioline moiety of the uncialamycin, a new enediyne natural product
Orbital Dependent Exchange-Only Methods for Periodic Systems
Various orbital-dependent exchange-only potentials are studied which exhibit
correct long-range asymptotic behaviour. We present the first application of
these potentials for polymers and by one of these potentials for molecules.
Kohn-Sham type calculations have been carried out for polyethylene in order to
make valuable comparison of these potentials with each other as well as with
Hartree-Fock and exchange-only LDA methods. The Kohn-Sham band gap obtained
with the optimized effective potetial method is corrected with the exchange
contribution to the derivative discontinuity of the exchange-correlation
potential. The corrected band gap obtained with the Slater's exchange potential
is 9.7 eV close to the experiment.Comment: 11 pages, 2 figures. Phys. Rev. B60, 1999, in pres
Contact Changes of Sheared Systems: Scaling, Correlations, and Mechanisms
We probe the onset and effect of contact changes in 2D soft harmonic particle
packings which are sheared quasistatically under controlled strain. First, we
show that in the majority of cases, the first contact changes correspond to the
creation or breaking of contacts on a single particle, with contact breaking
overwhelmingly likely for low pressures and/or small systems, and contact
making and breaking equally likely for large pressures and in the thermodynamic
limit. The statistics of the corresponding strains are near-Poissonian. The
mean characteristic strains exhibit scaling with the number of particles N and
pressure P, and reveal the existence of finite size effects akin to those seen
for linear response quantities. Second, we show that linear response accurately
predicts the strains of the first contact changes, which allows us to study the
scaling of the characteristic strains of making and breaking contacts
separately. Both of these show finite size scaling, and we formulate scaling
arguments that are consistent with the observed behavior. Third, we probe the
effect of the first contact change on the shear modulus G, and show in detail
how the variation of G remains smooth and bounded in the large system size
limit: even though contact changes occur then at vanishingly small strains,
their cumulative effect, even at a fixed value of the strain, are limited, so
that effectively, linear response remains well-defined. Fourth, we explore
multiple contact changes under shear, and find strong and surprising
correlations between alternating making and breaking events. Fifth, we show
that by making a link with extremal statistics, our data is consistent with a
very slow crossover to self averaging with system size, so that the
thermodynamic limit is reached much more slowly than expected based on finite
size scaling of elastic quantities or contact breaking strains
East African governments' responses to high cereal prices
This study analyses the responses of governments in four East African countries (Kenya, Tanzania, Uganda and Ethiopia) with respect to price formation and price transmission in the cereal sector. All four countries were confronted with high cereal prices in 2008. Government policies applied largely pursued consumer price reduction, but such short-term price policies did not encourage farmers to respond with increasing production, the more so because farmers were facing very high fertiliser prices. The report concludes by discussing policy options to help improve market functioning which supports agricultural productivity growt
Reverse Line Graph Construction: The Matrix Relabeling Algorithm MARINLINGA Versus Roussopoulos's Algorithm
We propose a new algorithm MARINLINGA for reverse line graph computation,
i.e., constructing the original graph from a given line graph. Based on the
completely new and simpler principle of link relabeling and endnode
recognition, MARINLINGA does not rely on Whitney's theorem while all previous
algorithms do. MARINLINGA has a worst case complexity of O(N^2), where N
denotes the number of nodes of the line graph. We demonstrate that MARINLINGA
is more time-efficient compared to Roussopoulos's algorithm, which is
well-known for its efficiency.Comment: 30 pages, 24 figure
An Application-Tailored MAC Protocol for Wireless Sensor Networks
We describe a data management framework suitable for wireless sensor networks that can be used to adapt the performance of a medium access control (MAC) protocol depending on the query injected into the network. The framework has a\ud
completely distributed architecture and only makes use of information available locally to capture information about network traffic patterns. It allows\ud
nodes not servicing a query to enter a dormant mode which minimizes transmissions and yet maintain an updated view of the network. We then introduce an Adaptive, Information-centric and Lightweight MAC\ud
(AI-LMAC) protocol that adapts its operation depending on the information presented by the framework. Our results demonstrate how transmissions are greatly reduced during the dormant mode. During the active mode, the MAC\ud
protocol adjusts fairness to match the expected requirements of the query thus reducing latency. Thus such a data management framework allows the MAC to operate more efficiently by tailoring its needs to suit the requirements of the application
Consistent description of nuclear charge radii and electric monopole transitions
A systematic study of energy spectra throughout the rare-earth region
(even-even nuclei from Ce to W) is carried out in the framework
of the interacting boson model (IBM), leading to an accurate description of the
spherical-to-deformed shape transition in the different isotopic chains. The
resulting IBM Hamiltonians are then used for the calculation of nuclear charge
radii (including isotope and isomer shifts) and electric monopole transitions
with consistent operators for the two observables. The main conclusion of this
study is that an IBM description of charge radii and electric monopole
transitions is possible for most of the nuclei considered but that it breaks
down in the tungsten isotopes. It is suggested that this failure is related to
hexadecapole deformation.Comment: 13 pages, 5 tables, 3 figures, accepted for publication in Physical
Review
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