  The coupling of single-particle motion and of vibrations in $^{11}_4$Be
produces dressed neutrons which spend only a fraction of the time in pure
single-particle states, and which weighting differently from the bare neutrons,
lead to parity inversion. The interaction of the two least bound neutrons in
the ground state of $^{12}_4$Be mediated by the $v_{14}$ Argonne
nucleon-nucleon potential and by the exchange of the phonon cloud, give rise to
a strongly correlated state, where the neutrons spend more than half of the
time in (s$^2$+d$^2$)-configurations, resulting in a breaking of the N=8 shell
closure.
