Shape coexistence and shape transition in self-conjugate nucleus 72Kr and the tensor force

Oblate-prolate shape-coexistence is well-known in the N = Z nucleus Kr-72. Furthermore, recent experimental data implies that there is a rapid shape transition at very low spins in this nucleus. We reinvestigate this problem by using large-scale shell-model calculations with the monopole interactions derived from the monopole-based universal force that contains the tensor force in the Hamiltonian. We show that in Kr-72, states with nucleon-quartet excitation from the pf shell to the g(9/2)-d(5/2) orbits, which favor large prolate deformation, compete with those having the pf shell as the main configuration with oblate deformation. These shapes can coexist if the two types of states reside closely in excitation energy. In Kr-72 the tensor force is found to provide precisely such a coexistence condition near the ground state. As the tensor effect changes dynamically with orbital occupation when the nucleus rotates, a rapid shape transition can occur.