Projected shell model analysis of structural evolution and chaoticity in fast-rotating nuclei

Abstract

Rotationally-induced chaotic motion in fast-rotating nuclei is investigated using the projected shell model with an aggressively extended multi-quasiparticle configuration space. The two-body Hamiltonian in separable forms with quadrupole–quadrupole plus pairing forces is diagonalized in superimposed states with more than 2000 angular momentum projected configurations. By taking 164Yb as an example, it is shown that the experimental energies and moment of inertia (MoI) for the yrast band can be clearly described. The variations in MoI are explained in terms of band crossings among the 2-quasiparticle (qp), 4-qp, 6-qp, and 8-qp bands, corresponding to successive pair-breakings caused by rotation. Moreover, the chaotic motion in different spin and excitation regions is studied by quantitatively analyzing the branching number. It is found that the degree of chaoticity increases monotonically with spin and excitation energy. At the highest spins, bands built by different numbers of quasiparticles, with their own characteristic rotational behavior originally at low spins, tend to rotate uniformly with the same rotational frequency, indicating an emergent nuclear collectivity appearing from an extremely chaotic system. The important role played by nucleon pairing in increasing the chaoticity is emphasized.