2,275 research outputs found
Calculation of stellar electron-capture cross sections on nuclei based on microscopic Skyrme functionals
A fully self-consistent microscopic framework for evaluation of nuclear
weak-interaction rates at finite temperature is introduced, based on Skyrme
functionals. The single-nucleon basis and the corresponding thermal occupation
factors of the initial nuclear state are determined in the finite-temperature
Skyrme Hartree-Fock model, and charge-exchange transitions to excited states
are computed using the finite-temperature RPA. Effective interactions are
implemented self-consistently: both the finite-temperature single-nucleon
Hartree-Fock equations and the matrix equations of RPA are based on the same
Skyrme energy density functional. Using a representative set of Skyrme
functionals, the model is applied in the calculation of stellar
electron-capture cross sections for selected nuclei in the iron mass group and
for neutron-rich Ge isotopes.Comment: 31 pages, 13 figures, submitted to Physical Review
Exotic modes of excitation in atomic nuclei far from stability
We review recent studies of the evolution of collective excitations in atomic
nuclei far from the valley of -stability. Collective degrees of freedom
govern essential aspects of nuclear structure, and for several decades the
study of collective modes such as rotations and vibrations has played a vital
role in our understanding of complex properties of nuclei. The multipole
response of unstable nuclei and the possible occurrence of new exotic modes of
excitation in weakly-bound nuclear systems, present a rapidly growing field of
research, but only few experimental studies of these phenomena have been
reported so far. Valuable data on the evolution of the low-energy dipole
response in unstable neutron-rich nuclei have been gathered in recent
experiments, but the available information is not sufficient to determine the
nature of observed excitations. Even in stable nuclei various modes of giant
collective oscillations had been predicted by theory years before they were
observed, and for that reason it is very important to perform detailed
theoretical studies of the evolution of collective modes of excitation in
nuclei far from stability. We therefore discuss the modern theoretical tools
that have been developed in recent years for the description of collective
excitations in weakly-bound nuclei. The review focuses on the applications of
these models to studies of the evolution of low-energy dipole modes from stable
nuclei to systems near the particle emission threshold, to analyses of various
isoscalar modes, those for which data are already available, as well as those
that could be observed in future experiments, to a description of
charge-exchange modes and their evolution in neutron-rich nuclei, and to
studies of the role of exotic low-energy modes in astrophysical processes.Comment: 123 pages, 59 figures, submitted to Reports on Progress in Physic
Bursts in the Chaotic Trajectory Lifetimes Preceding the Controlled Periodic Motion
The average lifetime () it takes for a randomly started trajectory
to land in a small region () on a chaotic attractor is studied. is
an important issue for controlling chaos. We point out that if the region
is visited by a short periodic orbit, the lifetime strongly deviates
from the inverse of the naturally invariant measure contained within that
region (). We introduce the formula that relates
to the expanding eigenvalue of the short periodic orbit
visiting .Comment: Accepted for publication in Phys. Rev. E, 3 PS figure
Excitation of Pygmy Dipole Resonance in neutron-rich nuclei via Coulomb and nuclear fields
We study the nature of the low-lying dipole strength in neutron-rich nuclei,
often associated to the Pygmy Dipole Resonance. The states are described within
the Hartree-Fock plus RPA formalism, using different parametrizations of the
Skyrme interaction. We show how the information from combined reactions
processes involving the Coulomb and different mixtures of isoscalar and
isovector nuclear interactions can provide a clue to reveal the characteristic
features of these states.Comment: 9 Pages, 8 figures, contribution to International Symposium On
Nuclear Physics, December 8-12, 2009,Bhabha Atomic Research Centre, Mumbai,
Indi
Incompressibility of finite fermionic systems: stable and exotic atomic nuclei
The incompressibility of finite fermionic systems is investigated using
analytical approaches and microscopic models. The incompressibility of a system
is directly linked to the zero-point kinetic energy of constituent fermions,
and this is a universal feature of fermionic systems. In the case of atomic
nuclei, this implies a constant value of the incompressibility in medium-heavy
and heavy nuclei. The evolution of nuclear incompressibility along Sn and Pb
isotopic chains is analyzed using global microscopic models, based on both
non-relativistic and relativistic energy functionals. The result is an almost
constant incompressibility in stable nuclei and systems not far from stability,
and a steep decrease in nuclei with pronounced neutron excess, caused by the
emergence of a soft monopole mode in neutron-rich nuclei.Comment: 7 pages, 5 figure
Transport, magnetic and superconducting properties of RuSr2RCu2O8 (R= Eu, Gd) doped with Sn
Ru{1-x}Sn{x}Sr2EuCu2O8 and Ru{1-x}Sn{x}Sr2GdCu2O8 have been comprehensively
studied by microwave and dc resistivity and magnetoresistivity and by the dc
Hall measurements. The magnetic ordering temperature T_m is considerably
reduced with increasing Sn content. However, doping with Sn leads to only
slight reduction of the superconducting critical temperature T_c accompanied
with the increase of the upper critical field B_c2, indicating an increased
disorder in the system and a reduced scattering length of the conducting holes
in CuO2 layers. In spite of the increased scattering rate, the normal state
resistivity and the Hall resistivity are reduced with respect to the pure
compound, due to the increased number of itinerant holes in CuO2 layers, which
represent the main conductivity channel. Most of the electrons in RuO2 layers
are presumably localized, but the observed negative magnetoresistance and the
extraordinary Hall effect lead to the conclusion that there exists a small
number of itinerant electrons in RuO layers that exhibit colossal
magnetoresistance.Comment: 10 pages, 9 figure
Nonabelian density functional theory
Given a vector space of microscopic quantum observables, density functional
theory is formulated on its dual space. A generalized Hohenberg-Kohn theorem
and the existence of the universal energy functional in the dual space are
proven. In this context ordinary density functional theory corresponds to the
space of one-body multiplication operators. When the operators close under
commutation to form a Lie algebra, the energy functional defines a Hamiltonian
dynamical system on the coadjoint orbits in the algebra's dual space. The
enhanced density functional theory provides a new method for deriving the group
theoretic Hamiltonian on the coadjoint orbits from the exact microscopic
Hamiltonian.Comment: 1 .eps figur
Low-lying dipole response in the Relativistic Quasiparticle Time Blocking Approximation and its influence on neutron capture cross sections
We have computed dipole strength distributions for nickel and tin isotopes
within the Relativistic Quasiparticle Time Blocking approximation (RQTBA).
These calculations provide a good description of data, including the
neutron-rich tin isotopes Sn. The resulting dipole strengths have
been implemented in Hauser-Feshbach calculations of astrophysical neutron
capture rates relevant for r-process nucleosynthesis studies. The RQTBA
calculations show the presence of enhanced dipole strength at energies around
the neutron threshold for neutron rich nuclei. The computed neutron capture
rates are sensitive to the fine structure of the low lying dipole strength,
which emphasizes the importance of a reliable knowledge of this excitation
mode.Comment: 15 pages, 4 figures, Accepted in Nucl. Phys.
Soft Dipole Modes in Neutron-rich Ni-isotopes in QRRPA
The soft dipole modes in neutron rich even-even Ni-isotopes are investigated
in the quasiparticle relativistic random phase approximation. We study the
evolution of strengths distribution, centroid energies of dipole excitation in
low-lying and normal GDR regions with the increase of the neutron excess. It is
found in the present study that the centroid energies of the soft dipole
strengths strongly depend on the thickness of neutron skin along with the
neutron rich even-even Ni-isotopes.Comment: 14 pages, 7 figure
A Transport and Microwave Study of Superconducting and Magnetic RuSr2EuCu2O8
We have performed susceptibility, thermopower, dc resistance and microwave
measurements on RuSr2EuCu2O8. This compound has recently been shown to display
the coexistence of both superconducting and magnetic order. We find clear
evidence of changes in the dc and microwave resistance near the magnetic
ordering temperature (132 K). The intergranular effects were separated from the
intragranular effects by performing microwave measurements on a sintered
ceramic sample as well as on a powder sample dispersed in an epoxy resin. We
show that the data can be interpreted in terms of the normal-state resistivity
being dominated by the CuO2 layers with exchange coupling to the Ru moments in
the RuO2 layers. Furthermore, most of the normal-state semiconductor-like
upturn in the microwave resistance is found to arise from intergranular
transport. The data in the superconducting state can be consistently
interpreted in terms of intergranular weak-links and an intragranular
spontaneous vortex phase due to the ferromagnetic component of the
magnetization arising from the RuO2 planes.Comment: 20 pages including 6 figures in pdf format. To be published in Phys.
Rev.
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