Low-energy dipole excitations in $^{20}$O with antisymmetrized molecular dynamics

Abstract

Low-energy dipole (LED) excitations in 20 O were investigated by variation after K-projection of deformation(β)-constraint antisymmetrized molecular dynamics combined with the generator coordinate method. We obtained two LED states, namely, the 1 − 1 state with one-proton excitation on the relatively weak deformation and the 1 − 2 state with a parity asymmetric structure of the normal deformation. The former is characterized by a toroidal dipole (TD) mode with vortical nuclear current, whereas the latter is associated with a low-energy E1 mode caused by surface neutron oscillation along the prolate deformation. The TD (vortical) and E1 modes separately appear as the K π = 1 − and K π = 0 − components of the deformed states, respectively, but couple with each other in the 1 − 1 and 1 − 2 states of 20 O because of K-mixing, and shape fluctuation. As a result of the mixing, TD and E1 transition strengths are fragmented into the 1 − 1 and 1 − 2 states. We also obtained the K π = 0 + , K π = 0 − , and K π = 1 − bands with cluster structures in the energy region higher than the LED states.