The role of virtual states in the appearance of two-neutron halo

The existence of loosely bound, spatially extended states in a two-particle plus core system is studied. The description is given in the Schroedinger picture by treating the interactions in the scattering length approximation. The treatment supports the statement that the most favourable case for the appearance of the two-neutron halo is the existence of large negative neutron-core scattering length (a virtual state) besides the n-n virtual state. (orig.)