Deformed neutron halos with shape decoupling in neutron-rich magnesium isotopes

摘要: Background: The exploration of neutron-rich nuclei far from stability has revealed exotic phenomena like the change of magic numbers, shape coexistence, and halo formation. Neutron-rich magnesium isotopes provide a pivotal testing ground for understanding how shell evolution, deformation, and continuum coupling collectively govern nuclear structure near the drip line. Purpose: This work aims to systematically investigate the ground-state properties and the emergence of deformed neutron halos in even-even magnesium isotopes34−44Mg, with a focus on the microscopic mechanisms driving shell closure quenching, deformation development, and halo formation. Methods: We employ the deformed relativistic mean-field theory combined with the complex momentum representation and BCS pairing (DRMF-CMR-BCS). This framework self-consistently treats deformation, pairing correlations, and continuum coupling, providing a unified description of bound, resonant, and continuum states. Calculations are performed using the NL3 effective interaction. Results: Our calculations reveal the microscopic mechanism for the collapse of the N = 20 and N = 28 shell closures, identifying it as a cooperation of monopole drift in key neutron orbitals (e.g., 1/2−1, 3/2−2) and the stabilization of prolate deformation. In 40,42,44Mg, we predict the universal emergence of deformed halos characterized by a striking shape decoupling: a prolate core coexists with an oblate halo. This halo is predominantly formed by low-angular-momentum orbitals, with the dominant contributor shifting from a narrow resonant state (3/2−2) in 40Mg to a weakly bound orbital (3/2−2) in 42Mg. The anomalous occupancy of narrow resonances underscores the important role of pairing, enhanced continuum coupling. Conclusions: The structure of neutron-rich Mg isotopes is governed by the intricate competition between single-particle energies, deformation, pairing, and the continuum. Our calculations offer clear, testable predictions for future rare-isotope beam experiments.