Exotic nuclear shape due to cluster formation at high angular momentum

It is shown, for the first time, how the exotic shapes due to cluster formation at high excitation energy and angular momentum are manifested through giant dipole resonance (GDR) strength function under the framework of the extended quantum molecular dynamics (EQMD) model. The results of EQMD calculation are compared with the existing experimental data of $^{32}\mathrm{S}$ and $^{28}\mathrm{Si}$ formed in the reactions $^{20}\mathrm{Ne}+^{12}\mathrm{C}$ and $^{16}\mathrm{O}+^{12}\mathrm{C}$, respectively, at high angular momenta. It is found that the EQMD predicts the general trend of the experimental GDR strength functions for $^{32}\mathrm{S}$ and $^{28}\mathrm{Si}$ by considering the ring or toroidal configuration, whereas the linear chain configurations with $\ensuremath{\alpha}$ clusters can reproduce the higher-energy peak in $^{32}\mathrm{S}$ and $^{28}\mathrm{Si}$. Thus, the direct signature of the cluster formation at high temperature and angular momentum is the observation of a GDR peak $\ensuremath{\approx}25\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ which cannot be predicted within the mean-field calculations. The present result highlights the role of $\ensuremath{\alpha}$ cluster states above the decay threshold, which is still an open field of investigation.