The coupling of single-particle motion and of vibrations in ${}_4{}^{11}\mathrm{Be}$ produces dressed neutrons which spend only a fraction of the time in pure single-particle states, and which weighing differently from the bare neutrons lead to parity inversion. The interaction of the two least bound neutrons in the ground state of ${}_4{}^{12}\mathrm{Be}$ mediated by the $v_{14}$ Argonne nucleon-nucleon potential and by the exchange of surface vibrations of the core ${}^{10}\mathrm{Be}$ gives rise to a strongly correlated state, where the two valence neutrons are distributed over $s^2,p^2$, and $d^2$ configurations, resulting in the breaking of the $N=8$ shell closure.

• Received 18 April 2003

DOI:https://doi.org/10.1103/PhysRevC.69.041302