Pairing and continuum effects on low-frequency quadrupole vibrations in deformed Mg isotopes close to the neutron drip line

Low-frequency quadrupole vibrational modes in deformed 36,38,40Mg close to the neutron drip line are studied by means of the quasiparticle-random-phase approximation based on the coordinate-space Hartree–Fock–Bogoliubov formalism. Strongly collective Kπ=0+ and 2+ excitation modes carrying 10–20 Weisskopf units in the intrinsic isoscalar quadrupole transition strengths are obtained at about 3 MeV. There are two reasons for the enhancement of the transition strengths. First, the quasiparticle wave functions generating these modes possess spatially very extended structure. The asymptotic selection rules characterizing the β and γ vibrations in stable deformed nuclei are thus strongly violated. Second, the dynamic pairing effects act strongly to enhance the collectivity of these modes. It is suggested that the lowest Kπ=0+ collective mode is a particularly sensitive indicator of the nature of pairing correlations in deformed nuclei close to the neutron drip line.