Structure change from shell-model-like ground state to cluster states

Coexistence features of cluster states and mean-field-type states are discussed in three nuclei, 20Ne, 44Ti, and 32S. It is demonstrated that AMD studies of these nuclei reproduce successfully many experimental data of the coexistence. According to AMD, the percentage of the 16O + α cluster component in the ground state of 20Ne is about 70 % and that of the 40Ca + α cluster componen of the ground state of 44Ti is about 40 %. These percentages decrease for high spin members of the ground rotational bands; for example 34 % and 25 % for the 6+ states of 20Ne and 44Ti, respectively. We see that, inspite of these rather small percentage numbers, the relative motions of 16O - α and 40Ca - α are excited to yield the excited rotational bands with much larger percentage numbers in 20Ne and 44Ti, respectively. Similar situation is seen in the superdeformed excited band and 16O + 16O molecular bands in 32S. The mechanism of the structure change from shell-model-like ground state to cluster states is discussed through the analysis of the monopole transitions between ground state and cluster states in 16O and 12C. Our arguments are based on the Bayman-Bohr theorem which says that the SU(3) shell model wave functions of the ground states have dual character in the sense that they are equivalently expressed by cluster model wave functions.