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 Mg-36,Mg-38,Mg-40 close to the neutron drip line are studied by means of the quasi particle-random-phase approximation based on the coordinate-space Hartree-Fock-Bogoliubov formalism. Strongly collective K-pi = 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 beta and gamma 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 K-pi = 0(+) collective mode is a particularly sensitive indicator of the nature of pairing correlations in deformed nuclei close to the neutron drip line. (c) 2006 Elsevier B.V. All rights reserved.