Parity inversion and breakdown of shell closure in Be isotopes

The coupling of single-particle motion and of vibrations in Be-11(4) 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 Be-12(4) mediated by the v(14) Argonne nucleon-nucleon potential and by the exchange of surface vibrations of the core Be-10 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.