Systematic Description of Yrast Superdeformed Bands in Even-Even Nuclei of the Mass-190 Region

Abstract

Yrast superdeformed bands for even-even nuclei of the mass-190 region are described by the projected shell model. Excellent agreement with available data for all isotopes is obtained. Our calculation of electromagnetic properties and pairing correlations provides a microscopic understanding of the observed gradual increase of dynamical moments of inertia with angular momentum in this mass region and suggests that for superdeformation it is not very meaningful to distinguish between Coriolis antipairing and gradual high-j orbital alignment. [S0031-9007(97)02692-6] PACS numbers: 21.10. Re, 21.30.Fe, 21.60.Cs, 27.80. + w A slowly rotating nuclear system can often be characterized by a fixed deformation with pairing correlation among nucleons. As the system rotates more rapidly, these 2 degrees of freedom will be modified by the rotation. Three simple consequences may be identified: (1) the nuclear deformation can vary during the rotation, a phenomenon known as the stretching effect [1]; (2) the Coriolis antipairing effect (CAP) [2], which is caused by the weakening pairing correlations across many orbitals due to the Coriolis force; and (3) rotation alignment [3], which emphasizes an alignment along the rotation axis of a pair in an orbital particularly susceptible to the Coriolis effect. All these effects can lead to a variation in moment of inertia (MoI).Generally, all of these effects may be expected in rotating nuclear systems, but in special cases one of them may dominate. For example, in rare-earth nuclei the observed sudden enhancement in the MoI associated with a backbend is typically dominated by rotation alignment of a nucleon pair from a high-j and low-V orbital [3], with CAP and stretching effects playing less important roles. However, for situations where the variation in the MoI is gradual, the measured g-ray energies are often not sufficient to distinguish the effects that change the MoI. Additional measurements of transition quadrupole moments, g factors, or nucleon pair transfer reactions can provide information to disentangle these contributions, but these are more difficult than energy measurements.The situation for superdeformed (SD) nuclei is less clear. In the SD nuclei of the mass-190 region, both kinematical ͑J ͑1͒ ͒ and dynamical ͑J ͑2͒ ͒ MoI for most SD bands exhibit a gradual increase as a function of increasing rotational frequency, with a more pronounced increase in J ͑2͒ . Thus, even though these are among the most deformed nuclei, they exhibit substantial deviation from rigid rotor behavior as the angular momentum increases. The usual understanding is that this behavior is caused by a gradual rotation alignment of pairs from high-j intruder orbitals [4]. There have been several approaches to the detailed calculation of SD bands and their properties [5][6][7][8][9][10][11][12]. Although these differ in particulars, a common feature is that the rotational degree of freedom is described by the cranking method. Thus no electromagnetic transition probabilities as a function of angular momen...

Shape-coexisting rotation in neutron-deficient Hg and Pb nuclei

“…To restore the broken rotational symmetry, the angularmomentum-projection (AMP) technique can be used. Hara and Sun developed a so-called projected shell model (PSM) [18,19] in which a deformed Nilsson basis is adopted and the angular momenta of model configurations are created by the AMP. The configurations are constructed in the quasiparticle (qp) scheme [18,19].…”

“…Hara and Sun developed a so-called projected shell model (PSM) [18,19] in which a deformed Nilsson basis is adopted and the angular momenta of model configurations are created by the AMP. The configurations are constructed in the quasiparticle (qp) scheme [18,19]. The existing PSM which we use considers zero-qp, two-qp, and four-qp configurations (for even-even nuclei) within three major Nilsson shells [18].…”

“…The existing PSM which we use considers zero-qp, two-qp, and four-qp configurations (for even-even nuclei) within three major Nilsson shells [18]. It has been shown that such PSM computation is fast and efficient [18,19]. However, the current PSM calculation is performed with a fixed deformation, and the result is dependent on what deformation is assumed.…”

SU(4) Model of High-Temperature Superconductivity: Manifestation of Dynamical Symmetry in Cuprates

“…Since this model is of the shell-model type based on single-particle degrees of freedom, and is known to give extremely good agreement with a broad range of experimental data (see, e.g., Ref. 15 ), this provides rather conclusive proof that these dynamical symmetries are strongly realized in the low-lying states of complex nuclei. This raises the issue of whether similar symmetries might be found in other complex many-body fermion systems such as those important in condensed matter.…”

“…(15), are variational parameters in the matrix representation that will be interpreted later as quasiparticle unoccupation and occupation amplitudes, respectively. The matrix (16) implements a quasiparticle transformation within the D-π pair space that is further constrained to preserve the SU(4) symmetry.…”