9 research outputs found
Parity-Projected Shell Model Monte Carlo Level Densities for fp-shell Nuclei
We calculate parity-dependent level densities for the even-even isotopes
58,62,66 Fe and 58 Ni and the odd-A nuclei 59 Ni and 65 Fe using the Shell
Model Monte Carlo method. We perform these calculations in the complete fp-gds
shell-model space using a pairing+quadrupole residual interaction. We find
that, due to pairing of identical nucleons, the low-energy spectrum is
dominated by positive parity states. Although these pairs break at around the
same excitation energy in all nuclei, the energy dependence of the ratio of
negative-to-positive parity level densities depends strongly on the particular
nucleus of interest. We find equilibration of both parities at noticeably lower
excitation energies for the odd-A nuclei 59 Ni and 65 Fe than for the
neighboring even-even nuclei 58 Ni and 66 Fe.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Pairing correlations in N~Z pf-shell nuclei
We perform Shell Model Monte Carlo calculations to study pair correlations in
the ground states of nuclei with masses A=48-60. We find that ,
proton-neutron correlations play an important, and even dominant
role, in the ground states of odd-odd nuclei, in agreement with
experiment. By studying pairing in the ground states of Fe, we
observe that the isovector proton-neutron correlations decrease rapidly with
increasing neutron excess. In contrast, both the proton, and trivially the
neutron correlations increase as neutrons are added.
We also study the thermal properties and the temperature dependence of pair
correlations for Mn and Fe as exemplars of odd-odd and even-even
nuclei. While for Fe results are similar to those obtained for
other even-even nuclei in this mass range, the properties of Mn at low
temperatures are strongly influenced by isovector neutron-proton pairing. In
coexistence with these isovector pair correlations, our calculations also
indicate an excess of isoscalar proton-neutron pairing over the mean-field
values. The isovector neutron-proton correlations rapidly decrease with
temperatures and vanish for temperatures above keV, while the isovector
correlations among like nucleons persist to higher temperatures. Related to the
quenching of the isovector proton-neutron correlations, the average isospin
decreases from 1, appropriate for the ground state, to 0 as the temperature
increases
Scalar quantum kinetic theory for spin-1/2 particles: mean field theory
Starting from the Pauli Hamiltonian operator, we derive a scalar quantum
kinetic equations for spin-1/2 systems. Here the regular Wigner two-state
matrix is replaced by a scalar distribution function in extended phase space.
Apart from being a formulation of principal interest, such scalar quantum
kinetic equation makes the comparison to classical kinetic theory
straightforward, and lends itself naturally to currently available numerical
Vlasov and Boltzmann schemes. Moreover, while the quasi-distribution is a
Wigner function in regular phase space, it is given by a Q-function in spin
space. As such, nonlinear and dynamical quantum plasma problems are readily
handled. Moreover, the issue of gauge invariance is treated. Applications (e.g.
ultra-dense laser compressed targets and their diagnostics), possible
extensions, and future improvements of the presented quantum statistical model
are discussed.Comment: 21 pages, 2 figure
Spin kinetic theory - quantum kinetic theory in extended phase space
The concept of phase space distribution functions and their evolution is used
in the case of en enlarged phase space. In particular, we include the intrinsic
spin of particles and present a quantum kinetic evolution equation for a scalar
quasi-distribution function. In contrast to the proper Wigner transformation
technique, for which we expect the corresponding quasi-distribution function to
be a complex matrix, we introduce a spin projection operator for the density
matrix in order to obtain the aforementioned scalar quasi-distribution
function. There is a close correspondence between this projection operator and
the Husimi (or Q) function used extensively in quantum optics. Such a function
is based on a Gaussian smearing of a Wigner function, giving a positive
definite distribution function. Thus, our approach gives a Wigner-Husimi
quasi-distribution function in extended phase space, for which the reduced
distribution function on the Bloch sphere is strictly positive. We also discuss
the gauge issue and the fluid moment hierarchy based on such a quantum kinetic
theory.Comment: 10 pages, to appear in Transport Theory and Statistical Physics,
proceedings of Vlasovia III, 200
Correlation times in the cubic bistable system.
We have carried out an experimental and theoretical study of the effect of external multiplicative white noise on the relaxation time of a general representation of a bistable system. The experiments were based on a bistable electronic analogue circuit; the calculations employed a new technique recently introduced by Jung and Risken. Satisfactory agreement is obtained between the results of these two very different approaches. We demonstrate that, by acting as an indicator of critical slowing down, the relaxation time can be used to provide a correct characterisation of a noise-induced transition