The negative parity states of ${}^{11}\mathrm{B}$ and ${}^{11}\mathrm{C}$ were studied based on the calculations of antisymmetrized molecular dynamics (AMD). The calculations reproduced well the experimental strengths of Gamov-Teller (GT), $M1$, and monopole transitions. We especially focused on the $3/2_3^{-}$ and $5/2_2^{-}$ states for which GT transition strengths were recently measured. The weak $M1$ and GT transitions for $3/2_3^{-}$ in ${}^{11}\mathrm{B}$ and ${}^{11}\mathrm{C}$ are described by a well-developed cluster structure of $2\alpha +t$ and $2\alpha {+}^3$He, respectively, while the strong transitions for $5/2_2^{-}$ is characterized by an intrinsic spin excitation with no cluster structure. It was found that the $3/2_3^{-}$ state is a dilute cluster state, and its features are similar to those of ${}^{12}\mathrm{C}(0_2^{+})$ which is considered to be a gas state of $3\alpha$ clusters.

• Received 2 November 2006

DOI:https://doi.org/10.1103/PhysRevC.75.024302