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Observed well-developed $alpha$ cluster states in $^{16}$O, located above the four $alpha$ threshold, are investigated from the viewpoint of Bose-Einstein condensation of $alpha$ clusters by using a field-theoretical superfluid cluster model in which the order parameter is defined. The experimental energy levels are reproduced well for the first time by calculation. In particular, the observed 16.7 MeV $0_7^+$ and 18.8 MeV $0_8^+$ states with low-excitation energies from the threshold are found to be understood as a manifestation of the states of the Nambu-Goldstone zero-mode operators, associated with the spontaneous symmetry breaking of the global phase, which is caused by the Bose-Einstein condensation of the vacuum 15.1 MeV $0^+_6$ state with a dilute well-developed $alpha$ cluster structure just above the threshold. This gives evidence of the existence of the Bose-Einstein condensate of $alpha$ clusters in $^{16}$O. It is found that the emergence of the energy level structure with a well-developed $alpha$ cluster structure above the threshold is robust, almost independently of the condensation rate of $alpha$ clusters under significant condensation rate. The finding of the mechanism why the level structure that is similar to $^{12}$C emerges above the four $alpha$ threshold in $^{16}$O reinforces the concept of Bose-Einstein condensation of $alpha$ clusters in addition to $^{12}$C.
Bose-Einstein condensation of alpha clusters in light and medium-heavy nuclei is studied in the frame of the field theoretical superfluid cluster model. The order parameter of the phase transition from the Wigner phase to the Nambu-Goldstone phase is
We investigate the linear-chain configurations of four-$alpha$ clusters in $^{16}$O using a Skyrme cranked Hartree-Fock method and discuss the relationship between the stability of such states and angular momentum. We show the existence of a region o
The equation of state and phase diagram of isospin-symmetric chemically equilibrated mixture of alpha particles and nucleons are studied in the mean-field approximation. The model takes into account the effects of Fermi and Bose statistics for nucleo
Recently, we have determined surface distributions of $alpha$ clusters in the ground state of $^{20}mathrm{Ne}$ from $alpha$-transfer cross sections, without investigating the properties of its excited states. In this paper we extend our comprehensio
The Bose-Einstein condensation of $alpha$ partciles in the multicomponent environment of dilute, warm nuclear matter is studied. We consider the cases of matter composed of light clusters with mass numbers $Aleq 4$ and matter that in addition these c