Important role of the spin-orbit interaction in forming the 1/2^+ orbital structure in Be isotopes

The structure of the second 0^+ state of ^{10}Be is investigated using a microscopic $\alpha+\alpha+n+n$ model based on the molecular-orbit (MO) model. The second 0^+ state, which has dominantly the (1/2^+)^2 configuration, is shown to have a particularly enlarged $\alpha-\alpha$ structure. The kinetic energy of the two valence neutrons occupying along the $\alpha-\alpha$ axis is reduced remarkably due to the strong $\alpha$ clustering and, simultaneously, the spin-orbit interaction unexpectedly plays important role to make the energy of this state much lower. The mixing of states with different spin structure is shown to be important in negative-parity states. The experimentally observed small-level spacing between 1^- and 2^- (~ 300 keV) is found to be an evidence of this spin-mixing effect. ^{12}{Be} is also investigated using $\alpha+\alpha+4n$ model, in which four valence neutrons are considered to occupy the (3/2^-)^2(1/2^+)^2 configuration. The energy surface of ^{12}Be is shown to exhibit similar characteristics, that the remarkable $\alpha$ clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2^-)^2(1/2^+)^2 configuration properly stronger in comparison with the closed p-shell (3/2^-)^2(1/2^-)^2 configuration.