Collapse Of N=28 Magicity In Exotic Mg-40-Probe Of Deformed Halo And 2n-Radioactivity At Mg Neutron Drip-Line

The exotic phenomenon of two-neutron halos and 2n-radioactivity are explored in the neutron-rich Mg-40,Mg-42,Mg-44 by employing various variants of the relativistic mean-field approach. The extended tail of spatial density distributions including the enhanced neutron radii and skin thickness, pairing correlations, single-particle spectrum and wave functions predict Mg-40,Mg-42,Mg-44 to be strong candidates for deformed neutron halos. Weakening of magicity at N = 28 plays a significant role in the existence of a weakly bound halo in Mg-40 which is currently the heaviest isotope of Mg accessible experimentally. Large deformation, mixing of f-p shell Nilsson orbitals and the valence neutron occupancy of p-states leads to a reduced centrifugal barrier and broader spatial density distributions that favour 2n-radioactivity in Mg-42,Mg-44.