Background: The 0₂^+ state of ^8He has been discovered by a recent experiment, which suggested a developed cluster structure of spatially correlated neutron pairs, called ``dineutrons'' (^2n). Purpose: We aim to investigate the structure of ^8He (0₁^+) and ^8He (0₂^+) to clarify the monopole excitation mode in ^8He system while focusing on the +^2n+^2n cluster structures and the breaking of ^2n clusters. Methods: We apply a microscopic cluster model with the generator coordinate method for the +^2n+^2n cluster and ^6He+^2n cluster dynamics. The p₃/₂-closure component, which is induced by the spin-orbit force, is also incorporated. Results: The present calculation reasonably reproduces the experimental data of the properties of ^8He, such as 2n and 4n separation energies, energy spectra, and radii. The spatially developed cluster structure of the 0₂^+ state is obtained. The 0₁^+ and 0₂^+ states have dominant components of +^2n+^2n 3-body cluster, but contain significant ^2n breaking components which contribute to the energy gain and size shrinking of the ^8He system. The monopole excitation in ^8He is regarded as a radial excitation, which is similar to that in ^12C. Conclusions: The +^2n+^2n cluster structures play a dominant role in ^8He (0₁, ₂^+), and the mixing of the dineutron breaking contributes to significant structural effects.
Nakagawa et al. (Fri,) studied this question.