  The detailed investigation of microscopic mechanisms leading to the formation
of bubble structures in the nuclei has been performed in the framework of
covariant density functional theory. The main emphasis of this study is on the
role of single-particle degrees of freedom and Coulomb interaction. In general,
the formation of bubbles lowers the Coulomb energy. However, in nuclei this
trend is counteracted by the quantum nature of the single-particle states: only
specific single-particle states with specific density profiles can be occupied
with increasing proton and neutron numbers. A significant role of central
classically forbidden region at the bottom of the wine bottle potentials in the
formation of nuclear bubbles (via primarily the reduction of the densities of
the $s$ states at $r=0$) has been revealed for the first time. Their formation
also depends on the availability of low-$l$ single-particle states for
occupation since single-particle densities represent the basic building blocks
of total densities. Nucleonic potentials disfavor the occupation of such states
in hyperheavy nuclei and this contributes to the formation of bubbles in such
nuclei. Additivity rule for densities has been proposed for the first time. It
was shown that the differences in the densities of bubble and flat density
nuclei follow this rule in the $A\approx 40$ mass region and in superheavy
nuclei with comparable accuracy. This strongly suggests the same mechanism of
the formation of central depression in bubble nuclei of these two mass regions.
Nuclear saturation mechanisms and self-consistency effects also affect the
formation of bubble structures. The detailed analysis of different aspects of
bubble physics strongly suggests that the formation of bubble structures in
superheavy nuclei is dominated by single-particle effects.

Nuclear Theory