Effects of collective modes on shell structure of 10Be and 24O core

We study the effects of collective modes on shell structure in nuclei near the neutron drip line close to ¹⁰Be and ²⁴O. Energy shifts in single-particle energies are evaluated by a particle-vibration coupling model. In the case of the ¹⁰Be core, the coupling to 2 ⁺ states is found to reduce the shell gap between 2 s_1/2 and 1 p_1/2 states. In the case of the ²⁴O core, the couplings to 3 ^- and 2 ⁺ states lead to a large energy gap between 2 s_1/2 and 1 d_3/2 states, in agreement with a recent experimental evidence of a new magic number at N=16 near the neutron drip line. In particular, the 3 ^- vibrations are found to play a crucial role to push down the energy of the 2 s_1/2 state.