arXiv subjects
Publications and source records attributed to M. Stoitsov.
Using relations between wave functions obtained in the framework of the relativistic mean field theory, we investigate the effects of pseudospin and spin symmetry breaking on the single nucleon wave functions in spherical nuclei. In our analysis, we apply both relativistic and non-relativistic self-consistent models as well as the harmonic oscillator model.
In certain neutron-rich Te isotopes, a decrease in the energy of the first excited 2+ state is accompanied by a decrease in the E2 strength to that state from the ground state, contradicting simple systematics and general intuition about quadrupole collectivity. We use a separable quadrupole-plus-pairing Hamiltonian and the quasiparticle random phase approximation to calculate energies, B(E2,0+ -> 2+) strengths, and g factors for the lowest 2+ states near 132Sn (Z >= 50). We trace the anomalous behavior in the Te isotopes to a reduced neutron pairing above the N = 82 magic gap.
Ground-state properties of neutron-rich $N \approx 28$ nuclei are described in the framework of Relativistic Hartree Bogoliubov (RHB) theory. The model uses the NL3 effective interaction in the mean-field Lagrangian, and describes pairing correlations by the pairing part of the finite range Gogny interaction D1S. Two-neutron separation energies and ground-state quadrupole deformations that result from fully self-consistent RHB solutions, are compared with available experimental data. The model predicts a strong suppression of the spherical N=28 shell gap for neutron-rich nuclei: the $1f7/2 \to fp$ core breaking results in deformed ground states. Shape coexistence is expected for neutron-rich Si, S and Ar isotopes.
The scaled harmonic oscillator basis (SHO) is derived by a local scaling-point transformation of the spherical harmonic oscillator radial wave functions. The unitary scaling transformation produces a basis with improved asymptotic properties. The SHO basis is employed in the solution of the relativistic Hartree-Bogoliubov (RHB) equations in configurational space. The model is applied in the self-consistent mean-field approximation to the description of the neutron halo in Ne isotopes. It is shown that an expansion of nucleon spinors and mean-field potentials in the SHO basis reproduces the asymptotic properties of neutron densities calculated by finite element discretization in the coordinate space. In the RHB description of neutron skins and halos, SHO bases in two or three dimensions can be a useful alternative to technically complicated solutions on a mesh in coordinate space.
We examine isovector and isoscalar neutron-proton correlations in an exactly solvable model based on the algebra SO(8). We look particularly closely at Gamow-Teller strength and double beta decay, both to isolate the effects of the two kinds of pairing and to test two approximation schemes: the renormalized neutron-proton QRPA (RQRPA) and generalized BCS theory. When isoscalar pairing correlations become strong enough a phase transition occurs and the dependence of the Gamow-Teller beta+ strength on isospin changes in a dramatic and unfamiliar way, actually increasing as neutrons are added to an N=Z core. Renormalization eliminates the well-known instabilities that plague the QRPA as the phase transition is approached, but only by unnaturally suppressing the isoscalar correlations. Generalized BCS theory, on the other hand, reproduces the Gamow-Teller strength more accurately in the isoscalar phase than in the usual isovector phase, even though its predictions for energies are equally good everywhere. It also mixes T=0 and T=1 pairing, but only on the isoscalar side of the phase transition.