Microscopic analysis of spherical to γ-soft shape transitions in Zn isotopes

The rapid transition between spherical and γ-soft shapes in Zn isotopes in the mass A ≤ 70 region, is analyzed using excitation spectra and collective wave functions obtained by diagonalization of a five-dimensional Hamiltonian for quadrupole vibrational and rotational degrees of freedom, with parameters determined by constrained self-consistent relativistic mean-field calculations for triaxial shapes. The microscopic potential energy surfaces, together with the characteristic collective observables, illustrate a rapid transition from near spherical shape at the N = 40 subshell, to γ-soft deformed shapes for lighter isotopes. The calculated spectra display fingerprints of a second-order shape phase transition that can be approximately described by the E(5) analytic solution.