  $\alpha$-cluster correlations in the ground states of $^{12}$C and $^{16}$O are studied. Because of the $\alpha$ correlations, the intrinsic states of $^{12}$C and $^{16}$O have triangle and tetrahedral shapes, respectively. The deformations are regarded as spontaneous symmetry breaking of rotational invariance, and the resultant oscillating surface density is associated with a density wave (DW) state caused by the instability of Fermi surface with respect to a kind of $1p$-$1h$ correlations. To discuss the symmetry breaking between uniform density states and the oscillating density state, a schematic model of a few clusters on a Fermi gas core in a one-dimensional finite box was introduced. The model analysis suggests structure transitions from a Fermi gas state to a DW-like state via a BCS-like state, and to a Bose Einstein condensation (BEC)-like state depending on the cluster size relative to the box size. It was found that the oscillating density in the DW-like state originates in Pauli blocking effects. 
alpha-cluster correlations and symmetry breaking in light nuclei