65 research outputs found
Magnetic dipole excitations in nuclei: elementary modes of nucleonic motion
The nucleus is one of the most multi-faceted many-body systems in the
universe. It exhibits a multitude of responses depending on the way one
'probes' it. With increasing technical advancements of beams at the various
accelerators and of detection systems the nucleus has, over and over again,
surprised us by expressing always new ways of 'organized' structures and layers
of complexity. Nuclear magnetism is one of those fascinating faces of the
atomic nucleus we discuss in the present review. We shall not just limit
ourselves to presenting the by now very large data set that has been obtained
in the last two decades using various probes, electromagnetic and hadronic
alike and that presents ample evidence for a low-lying orbital scissors mode
around 3 MeV, albeit fragmented over an energy interval of the order of 1.5
MeV, and higher-lying spin-flip strength in the energy region 5 - 9 MeV in
deformed nuclei, nor to the presently discovered evidence for low-lying
proton-neutron isovector quadrupole excitations in spherical nuclei. To the
contrary, we put the experimental evidence in the perspectives of understanding
the atomic nucleus and its various structures of well-organized modes of motion
and thus enlarge our discussion to more general fermion and bosonic many-body
systems.Comment: 59 pages, 59 figures, accepted for publication in Rev. Mod. Phys
Microscopic calculation of symmetry projected nuclear level densities
We present a quantum Monte Carlo method with exact projection on parity and angular momentum that is free of a sign problem for seniority-conserving nuclear interactions. This technique allows the microscopic calculation of angular momentum and parity-projected nuclear level densities. We present results for the Fe-55, Fe-56, and Fe-57 isotopes. Signatures of the pairing phase transition are observed in the angular momentum distribution of the nuclear level density
Cross-sections for neutrino-nucleus interactions on and
We calculate cross sections for neutral current quasi-elastic neutrino-nucleus scattering within a continuum RPA model, based on a Green's function approach. As residual interaction a Skyrme force is used. The unperturbed single particle wave functions are generated using either a Woods-Saxon potential or a Hartree-Fock calculation. These calculations have interesting applications. Neutrinos play an important role in supernova nucleosynthesis. To obtain more information about these processes, cross sections are folded with a Fermi-Dirac distribution with temperatures of approximately 10 9 K
Cross-sections for neutral-current neutrino-nucleus interactions: applications for C and O
We calculate cross sections for neutral current quasi-elastic neutrino-nucleus scattering within a continuum RPA model, based on a Green's function approach. As residual interaction a Skyrme force is used. The unperturbed single particle wave functions are generated using either a Woods-Saxon potential or a Hartree-Fock calculation. These calculations have interesting applications. Neutrinos play an important role in supernova nucleosynthesis. To obtain more information about these processes, cross sections are folded with a Fermi-Dirac distribution with temperatures of approximately 10 K
Optimal Monte Carlo Updating
Based on Peskun's theorem it is shown that optimal transition matrices in
Markov chain Monte Carlo should have zero diagonal elements except for the
diagonal element corresponding to the largest weight. We will compare the
statistical efficiency of this sampler to existing algorithms, such as
heat-bath updating and the Metropolis algorithm. We provide numerical results
for the Potts model as an application in classical physics. As an application
in quantum physics we consider the spin 3/2 XY model and the Bose-Hubbard model
which have been simulated by the directed loop algorithm in the stochastic
series expansion framework.Comment: 6 pages, 5 figures, replaced with published versio
SU(4) symmetry in the extended proton-neutron interacting boson model: multiplets and symmetry breaking
The manifestation of symmetry within an interacting boson model including particle-like and hole-like - and -bosons is shown for light nuclei around the Z=N=8 shell. We also present a consistent description of the particle-hole (intruder spin or spin) multiplets in the Extended Interacting Boson Model (EIBM) and of - ( spin) multiplets in the IBM-2 as a breaking of this symmetry
Bosons Confined in Optical Lattices: the Numerical Renormalization Group revisited
A Bose-Hubbard model, describing bosons in a harmonic trap with a
superimposed optical lattice, is studied using a fast and accurate variational
technique (MF+NRG): the Gutzwiller mean-field (MF) ansatz is combined with a
Numerical Renormalization Group (NRG) procedure in order to improve on both.
Results are presented for one, two and three dimensions, with particular
attention to the experimentally accessible momentum distribution and possible
satellite peaks in this distribution. In one dimension, a comparison is made
with exact results obtained using Stochastich Series Expansion.Comment: 10 pages, 15 figure
Electric monopole transitions from low energy excitations in nuclei
Electric monopole (E0) properties are studied across the entire nuclear mass surface. Besides an introductory discussion of various model results (shell model, geometric vibrational and rotational models, algebraic models), we point out that many of the largest E0 transition strengths, (E0), are associated with shape mixing. We discuss in detail the manifestation of E0 transitions and present extensive data for~: single-closed shell nuclei, vibrational nuclei, well-deformed nuclei, nuclei that exhibit sudden ground-state changes, and nuclei that exhibit shape coexistence and intruder states. We also give attention to light nuclei, odd-A nuclei, and illustrate a suggested relation between (E0) and isotopic shifts
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