633,141 research outputs found
Partial and Quasi Dynamical Symmetries in Nuclei
One of the interesting aspects in the study of atomic nuclei is the
strikingly regular behaviour many display in spite of being complex
quantum-mechanical systems, prompting the universal question of how regularity
emerges out of complexity. It is often conjectured that symmetries play a
pivotal role in our understanding of this emerging simplicity. But most
symmetries are likely to be broken, partial or both. Under such more realistic
conditions, does the concept of symmetry still provide a basis for our
understanding of regularity? I suggest that this requires the enlarged concepts
of partial and quasi dynamical symmetry.Comment: 5 pages, 5 figures, accepted for publication in Nuclear Physics New
Neutron-proton pairs in nuclei
A review is given of attempts to describe nuclear properties in terms of
neutron--proton pairs that are subsequently replaced by bosons. Some of the
standard approaches with low-spin pairs are recalled but the emphasis is on a
recently proposed framework with pairs of neutrons and protons with aligned
angular momentum. The analysis is carried out for general and applied to
nuclei in the and shells.Comment: 16 figures, 4 tables, accepted for publication in Int. J. of Modern
Physics
Seniority in quantum many-body systems
The use of the seniority quantum number in many-body systems is reviewed. A
brief summary is given of its introduction by Racah in the context of atomic
spectroscopy. Several extensions of Racah's original idea are discussed:
seniority for identical nucleons in a single- shell, its extension to the
case of many, non-degenerate shells and to systems with neutrons and
protons. To illustrate its usefulness to this day, a recent application of
seniority is presented in Bose--Einstein condensates of atoms with spin.Comment: 16 pages, 1 figure, accepted for publication in the Proceedings of
The American Institute of Physic
Cross-sectional river shapes: A variational discharge-resistance formulation
Cross-sectional river shapes were obtained from a variational principle: minimizing the bed friction for a given discharge and a given maximum lateral bed slope (angle of repose). The optimal shape is found to be independent of both the exponent in the friction law adopted and the value of the discharge, but it does depend on the angle of repose. The optimal profile is a single stream; for braided rivers the solution is suboptimal
An infrared investigation of the catalyst formation in the system Ni(acac)2, C3H4, (iBu)3AL for propadiene polymerization
The catalyst formation in the system Ni(acac)2, C3H4, (iBu)3Al was investigated by means of infrared spectroscopy. It was found that the Al(acac)3 and (iBu)2Al(acac) formed are both by-products of catalyst formation without a catalytic activity. Ni(acac)2 loses its acac groups forming the unstable (iBu)2Ni compound; without C3H4 being present, this compound disproportionates to Ni metal and isobutane and isobutene. In the presence of C3H4 an allyl-nickel complex is formed, which reacts with (iBu)3Al to give the actual catalyst, possibly a bimetallic allyl—nickel-aluminium complex. Catalysts such as Ni(acac)2, C3H4, (iBu)3Al and (πC3H5)2Ni with or without (iBu)3Al all selectively give 1, 2, 1, 2-polypropadiene. A Lewis base like pyridine not only decreases the polymerization rate but also changes the selectivity towards the formation of 1, 2, 2, 1-polymer
On the origin of exponential growth in induced earthquakes in Groningen
The Groningen gas field shows exponential growth in earthquakes event counts
around a magnitude M1 with a doubling time of 6-9 years since 2001. This
behavior is identified with dimensionless curvature in land subsidence, which
has been evolving at a constant rate over the last few decades {essentially
uncorrelated to gas production.} We demonstrate our mechanism by a tabletop
crack formation experiment. The observed skewed distribution of event
magnitudes is matched by that of maxima of event clusters with a normal
distribution. It predicts about one event \,M5 per day in 2025, pointing to
increasing stress to human living conditions.Comment: 12 pages, 7 figures, to appear in Earthquakes and Structure
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