2,090 research outputs found
Intermediate energy Coulomb excitation as a probe of nuclear structure at radioactive beam facilities
The effects of retardation in the Coulomb excitation of radioactive nuclei in
intermediate energy collisions (Elab ~100 MeV/nucleon) are investigated. We
show that the excitation cross sections of low-lying states in 11Be,
{38,40,42}S and {44,46}Ar projectiles incident on gold and lead targets are
modified by as much as 20% due to these effects. The angular distributions of
decaying gamma-rays are also appreciably modified.Comment: 21 pages, 3 figures, Phys. Rev. C, in pres
Coulomb Excitation of Multi-Phonon Levels of the Giant Dipole Resonance
A closed expression is obtained for the cross-section for Coulomb excitation
of levels of the giant dipole resonance of given angular momentum and phonon
number. Applications are made to the Goldhaber-Teller and Steinwedel-Jensen
descriptions of the resonance, at non-relativistic and relativistic bombarding
energies.Comment: 16 pages, 5 figure
Semiclassical treatment of fusion processes in collisions of weakly bound nuclei
We describe a semiclassical treatment of nuclear fusion reactions involving
weakly bound nuclei. In this treatment, the complete fusion probabilities are
approximated by products of two factors: a tunneling probability and the
probability that the system is in its ground state at the strong absorption
radius. We investigate the validity of the method in a schematic two-channel
application, where the channels in the continuum are represented by a single
resonant state. Comparisons with full coupled-channels calculations are
performed. The agreement between semiclassical and quantal calculations isquite
good, suggesting that the procedure may be extended to more sophisticated
discretizations of the continuum.Comment: 11 pages, 5 figure
Coulomb corrected eikonal description of the breakup of halo nuclei
The eikonal description of breakup reactions diverges because of the Coulomb
interaction between the projectile and the target. This divergence is due to
the adiabatic, or sudden, approximation usually made, which is incompatible
with the infinite range of the Coulomb interaction. A correction for this
divergence is analysed by comparison with the Dynamical Eikonal Approximation,
which is derived without the adiabatic approximation. The correction consists
in replacing the first-order term of the eikonal Coulomb phase by the
first-order of the perturbation theory. This allows taking into account both
nuclear and Coulomb interactions on the same footing within the computationally
efficient eikonal model. Excellent results are found for the dissociation of
11Be on lead at 69 MeV/nucleon. This Coulomb Corrected Eikonal approximation
provides a competitive alternative to more elaborate reaction models for
investigating breakup of three-body projectiles at intermediate and high
energies.Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev.
Free energies of crystalline solids: a lattice-switch Monte Carlo method
We present a method for the direct evaluation of the difference between the
free energies of two crystalline structures, of different symmetry. The method
rests on a Monte Carlo procedure which allows one to sample along a path,
through atomic-displacement-space, leading from one structure to the other by
way of an intervening transformation that switches one set of lattice vectors
for another. The configurations of both structures can thus be sampled within a
single Monte Carlo process, and the difference between their free energies
evaluated directly from the ratio of the measured probabilities of each. The
method is used to determine the difference between the free energies of the fcc
and hcp crystalline phases of a system of hard spheres.Comment: 5 pages Revtex, 3 figure
Quantum calculations of Coulomb reorientation for sub-barrier fusion
Classical mechanics and Time Dependent Hartree-Fock (TDHF) calculations of
heavy ions collisions are performed to study the rotation of a deformed nucleus
in the Coulomb field of its partner. This reorientation is shown to be
independent on charges and relative energy of the partners. It only depends
upon the deformations and inertias. TDHF calculations predict an increase by
30% of the induced rotation due to quantum effects while the nuclear
contribution seems negligible. This reorientation modifies strongly the fusion
cross-section around the barrier for light deformed nuclei on heavy collision
partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.Comment: accepted for publication in Physical Review Lette
Accuracy of B(E2; 0+ -> 2+) transition rates from intermediate-energy Coulomb excitation experiments
The method of intermediate-energy Coulomb excitation has been widely used to
determine absolute B(E2; 0+ -> 2+) quadrupole excitation strengths in exotic
nuclei with even numbers of protons and neutrons. Transition rates measured
with intermediate-energy Coulomb excitation are compared to their respective
adopted values and for the example of 26Mg to the B(E2; 0+ -> 2+) values
obtained with a variety of standard methods. Intermediate-energy Coulomb
excitation is found to have an accuracy comparable to those of long-established
experimental techniques.Comment: to be published in Phys. Rev.
Quantal Brownian Motion - Dephasing and Dissipation
We analyze quantal Brownian motion in dimensions using the unified model
for diffusion localization and dissipation, and Feynman-Vernon formalism. At
high temperatures the propagator possess a Markovian property and we can write
down an equivalent Master equation. Unlike the case of the
Zwanzig-Caldeira-Leggett model, genuine quantum mechanical effects manifest
themselves due to the disordered nature of the environment. Using Wigner
picture of the dynamics we distinguish between two different mechanisms for
destruction of coherence. The analysis of dephasing is extended to the low
temperature regime by using a semiclassical strategy. Various results are
derived for ballistic, chaotic, diffusive, both ergodic and non-ergodic motion.
We also analyze loss of coherence at the limit of zero temperature and clarify
the limitations of the semiclassical approach. The condition for having
coherent effect due to scattering by low-frequency fluctuations is also pointed
out. It is interesting that the dephasing rate can be either larger or smaller
than the dissipation rate, depending on the physical circumstances.Comment: LaTex, 23 pages, 4 figures, published vesio
Magneto-Transport in the Two-Dimensional Lorentz Gas
We consider the two-dimensional Lorentz gas with Poisson distributed hard
disk scatterers and a constant magnetic field perpendicular to the plane of
motion. The velocity autocorrelation is computed numerically over the full
range of densities and magnetic fields with particular attention to the
percolation threshold between hopping transport and pure edge currents. The
Ohmic and Hall conductance are compared with mode-coupling theory and a recent
generalized kinetic equation valid for low densities and small fields. We argue
that the long time tail as persists for non-zero magnetic field.Comment: 7 pages, 14 figures. Uses RevTeX and epsfig.sty. Submitted to
Physical Review
Genetic embedded matching approach to ground states in continuous-spin systems
Due to an extremely rugged structure of the free energy landscape, the
determination of spin-glass ground states is among the hardest known
optimization problems, found to be NP-hard in the most general case. Owing to
the specific structure of local (free) energy minima, general-purpose
optimization strategies perform relatively poorly on these problems, and a
number of specially tailored optimization techniques have been developed in
particular for the Ising spin glass and similar discrete systems. Here, an
efficient optimization heuristic for the much less discussed case of continuous
spins is introduced, based on the combination of an embedding of Ising spins
into the continuous rotators and an appropriate variant of a genetic algorithm.
Statistical techniques for insuring high reliability in finding (numerically)
exact ground states are discussed, and the method is benchmarked against the
simulated annealing approach.Comment: 17 pages, 12 figures, 1 tabl
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