  Giant neutron halos in medium-heavy nuclei are studied in the framework of
the Hartree-Fock-Bogoliubov (HFB) approach with Skyrme interactions. The
appearance of such structures depends sensitively on the effective interaction
adopted. This is illustrated by comparing the predictions of SLy4 and SkI4 in
the Ca and Zr isotopic chains. The former force gives no halo effect, the
latter predicts a neutron halo in the Zr chain with A>122 due to the weakly
bound orbitals 3p1/2 and 3p3/2. The structure of the halo is analyzed in terms
of the occupation probabilities of these orbitals and their partial
contributions to the neutron density. The anti-halo effect in Ni and Zr
isotopes is also discussed by comparing the occupation probabilities of
Hartree-Fock neutron single-particle states near the Fermi energy with the
corresponding HFB quasiparticle states.
