Exotic structure in light neutron-rich nuclei

In this presentation I discussed two subjects. One is the persistence of threefold symmetry in the ground state of C-12. Recently D-3h symmetry has been established in C-12, which reflects the geometric symmetry of the three a particles. Although the spin-orbit interaction plays a significant role and this interaction breaks the a clusters, we show that threefold symmetry of C-12 is still there. We use AQCM approach and discuss that inclusion of spin-orbit interaction, which is absent in the conventional microscopic a cluster models, is possible keeping the threefold symmetry. The second subject is the appearance of rod shape in C isotopes, which has been investigated in the framework of the cranking covariant density functional theory. The relationship between the stability of such states and the spin and isospin degrees of freedom is discussed; adding valence neutrons and rotating the system. These two effects stabilize the rod shape, and in addition, their coherent effect has been found; the sigma-orbits (parallel to the symmetry axis) of the valence neutrons, which enhances the rod shape, is lowered by the rotation of the system, and this sigma-orbit pulls down the single particle energies of protons with linear configuration owing to the proton-neutron interaction effect.