Alpha-cluster structure and density wave in oblate nuclei

TL;DR

This paper explores the connection between pentagon and triangle nuclear shapes in Si-28 and C-12 and their relation to density waves, using molecular dynamics and cluster models to explain shape formation and symmetry breaking.

Contribution

It demonstrates that specific nuclear shapes are linked to density waves and symmetry breaking, providing a unified understanding of cluster structures and shape coexistence in oblate nuclei.

Findings

Pentagon and triangle shapes correspond to specific negative-parity bands.

Density waves are enhanced in Z=N nuclei due to proton-neutron coherence.

Shape formations can be explained by spontaneous symmetry breaking of oblate states.

Abstract

Pentagon and triangle shapes in Si-28 and C-12 are discussed in relation with nuclear density wave. In the antisymmetrized molecular dynamics calculations, the $K^\pi=5^-$ band in Si-28 and the $K^\pi=3^-$ band in C-12 are described by the pentagon and triangle shapes, respectively. These negative-parity bands can be interpreted as the parity partners of the $K^\pi=0^+$ ground bands and they are constructed from the parity-asymmetric-intrinsic states. The pentagon and the triangle shapes originate in 7alpha and 3alpha cluster structures, respectively. In a mean-field picture, they are described also by the static one-dimensional density wave at the edge of the oblate states. In analysis with ideal alpha cluster models using Brink-Bloch cluster wave functions and that with a simplified model, we show that the static edge density wave for the pentagon and triangle shapes can be understood by spontaneous breaking of axial symmetry, i.e., the instability of the oblate states with respect to the edge density wave. The density wave is enhanced in the Z=N nuclei due to the proton-neutron coherent density waves, while it is suppressed in Z\ne N nuclei.