  The effects of the phonon-phonon coupling on the beta-decay rates of
neutron-rich nuclei are studied in a microscopic model based on Skyrme-type
interactions. The approach uses a finite-rank separable approximation of the
Skyrme-type particle-hole (p-h) residual interaction. Very large
two-quasiparticle spaces can thus be treated. A redistribution of the
Gamow-Teller (G-T) strength is found due to the tensor correlations and the
2p-2h fragmentation of G-T states. As a result, the beta-decay half-lives are
decreased significantly. Using the Skyrme interaction SGII together with a
volume-type pairing interaction we illustrate this reduction effect by
comparing with available experimental data for the Ni isotopes and neutron-rich
N=50 isotones. We give predictions for 76Fe and 80Ni in comparison with the
case of the doubly-magic nucleus 78Ni which is an important waiting point in
the r-process.
