Doubly-deformed bands in doubly-magic Ni-56

The spherical-deformed shape coexistence of the doubly-magic Ni-56 nucleus is studied by the Monte Carlo shell model based on the Quantum Monte Carlo Diagonalization method. The coexistence of spherical yrast, first prolate d eformed and second deformed states is described by the full pf- and pfg(9/2 )-shell calculations. The states of the first prolate deformed band are sho wn to be comprised primarily of four-particle and four-hole excitations on top of the correlated ground state. The second deformed band has negative p arity states and can be described by including the g(9/2) degree of freedom .