Dipole response in deformed halo nuclei 42Mg and 44Mg

The quasiparticle finite amplitude method based on the deformed relativistic Hartree–Bogoliubov theory in continuum has been developed for the noncharge-exchange multipole response. Taking neutron-rich magnesium isotopes as examples, the isovector electric dipole response, especially in the low-lying region, is studied. It is found that the low-energy dipole strength increases with neutron number and becomes notably enhanced in the predicted deformed halo nuclei 42 Mg and 44 Mg. In these isotopes, the K π = 1 − states below 3 MeV are dominated by transitions from the “halo” part of the single-neutron orbitals. Their transition densities reveal a low-frequency, out-of-phase oscillation between the neutron halo and the core. These results provide a microscopic picture for the soft dipole resonance in 42 Mg and 44 Mg.