Shape transitions in exotic Si and S isotopes and tensor-force-driven Jahn-Teller effect

We show how shape transitions in the neutron-rich exotic Si and S isotopes occur in terms of shell-model calculations with a newly constructed Hamiltonian based on VMU interaction. We first compare the calculated spectroscopic-strength distributions for the proton 0d5/2,3/2 and 1s1/2 orbitals with results extracted from a ⁴⁸Ca(e,e^'p) experiment to show the importance of the tensor-force component of the Hamiltonian. Detailed calculations for the excitation energies, $B(E2)$, and two-neutron separation energies for the Si and S isotopes show excellent agreement with experimental data. The potential-energy surface exhibits rapid shape transitions along the isotopic chains towards $N=28$ that are different for Si and S. We explain the results in terms of an intuitive picture by involving a Jahn-Teller-type effect that is sensitive to the tensor-force-driven shell evolution. The closed subshell nucleus ⁴²Si is a particularly good example of how the tensor-force-driven Jahn-Teller mechanism leads to a strong oblate rather than a spherical shape.