  Experimental energy spectrum and B(E2) values in 68Ni and 90Zr indicate a
double-magic character in these neutron-rich nuclei with N or Z=40. The data
nevertheless do not show any pronounced irregularity in two-nucleon separation
energy. To understand the underlying physics, we carry out both shell-model and
mean-field calculations. The shell-model calculation can well reproduce all the
observations. It is understood from the mean-field results for 68Ni that the
shell gap at N=40 disappears due to dynamical correlations of the isovector J=0
pairing interaction. In 90Zr, however, such a dynamic process with the J=0
pairing appears not important because of the strong contribution of the J>0
interaction. We study also level schemes in the Ni isotopes and N=50 isotones.
We predict a new band built on the $0_{2}^{+}$ state in both 68Ni and 90Zr. The
states of this band are dominated by two-particle-two-hole excitations from the
fp-shell to the intruder g_{9/2} orbit.
