Low-energy dipole excitation modes in $^{10}$Be

Abstract

Low-energy dipole excitations in ${}^{10}\textrm{Be}$ were investigated based on the generator coordinate method with $\alpha + {}^6\textrm{He}$ and $\alpha + \alpha + 2n$ cluster models. We obtained three $1^-$ states in $E_{x}<15$ MeV, which show different characters in the $E1$, compressive dipole, and toroidal dipole transition strengths. We found the strong toroidal dipole transition in the $1_1^-$ state, remarkable $E1$ strength in the $1^-_2$ state, and the strong $E1$ and compressive dipole strength in the $1^-_3$ state. The $1^-_1$ is described by the $\alpha + {}^6\textrm{He}$ cluster structure, whereas the $1^-_2$ and $1^-_3$ states are understood by three-body excitations of the $\alpha + \alpha + 2n$ clustering.