Rapid structural change in low-lying states of neutron-rich Sr and Zr isotopes

The rapid structural change in low-lying collective excitation states of neutron-rich Sr and Zr isotopes is studied by solving a five-dimensional collective Hamiltonian with parameters determined from both relativistic mean-field and nonrelativistic Skyrme-Hartree-Fock calculations using the PC-PK1 and SLy4 forces, respectively. Pair correlations are treated in the BCS method with either a separable pairing force or a density-dependent zero-range force. The isotope shifts, excitation energies, and electric monopole and quadrupole transition strengths are calculated and compared with corresponding experimental data. The calculated results with both the PC-PK1 and the SLy4 forces exhibit a picture of spherical-oblate-prolate shape transition in neutron-rich Sr and Zr isotopes. However, compared with the experimental data, the PC-PK1 (or SLy4) force predicts a more moderate (or dramatic) change in most of the collective properties around N = 60. The underlying microscopic mechanism responsible for the rapid transition is discussed.