Skyrme-QRPA calculations for low-lying excitation modes in deformed neutron-rich nuclei

Low-frequency modes of excitation in deformed neutron-rich nuclei are studied by means of the quasiparticle random-phase approximation on the Skyrme-Hartree-Fock-Bogoliubov mean field. We investigate the microscopic structure of the soft K-pi = 0(+) modes systematically in neutron-rich magnesium isotopes with N = 22, 24, 26 and 28 close to the drip line, and it is found that the strong collectivity in Mg-34 and Mg-40 is acquired due to the coherent coupling between the beta vibration and the pairing vibration of neutrons. Microscopic structure of the K-pi = 2(+) modes changes gradually associated with the location of the Fermi level of neutrons, and it is found that the proton particle-hole excitation generating the gamma-vibrational mode in Mg-24 continues to play a key role in the near-drip-line nucleus Mg-40. The low-frequency octupole excitations are also investigated and the microscopic mechanism for the enhancement of transition strengths is discussed.