Toroidal, compressive, and $E1$ properties of low-energy dipole modes in $^{10}$Be

Abstract

We studied dipole excitations in $^{10}$Be based on an extended version of the antisymmetrized molecular dynamics, which can describe 1p-1h excitations and large amplitude cluster modes. Toroidal and compressive dipole operators are found to be good proves to separate the low-energy and high-energy parts of the isoscalar dipole excitations, respectively. Two low-energy $1^-$ states, the toroidal dominant $1^-_1$ state at $E\sim 8$ MeV and the $E1$ dominant $1^-_2$ state at $E\sim 16$ MeV, were obtained. By analysis of transition current densities, the $1^-_1$ states is understood as a toroidal dipole mode with exotic toroidal neutron flow caused by rotation of a deformed $^6\textrm{He}$ cluster, whereas the $1^-_2$ state is regarded as a neutron-skin oscillation mode, which are characterized by surface neutron flow with inner isoscalar flow caused by the surface neutron oscillation against the $2\alpha$ core.