Intruder structure in ¹¹Li from an ab initio perspective

The ground state of ¹¹Li has a well-known neutron halo structure, which, in a shell-model interpretation, arises from intruder contributions, in which neutrons are excited across the N=8 shell closure. Ab initio no-core configuration interaction (NCCI) calculations, for light nuclei in general, predict a low-lying spectrum which includes both normal (0ℏω) and intruder (2ℏω) states. The convergence of intruder states can be a challenge, but, in calculations with sufficiently soft interactions, the intruder states approach their low-lying physical energies. In ¹¹Li, we find that the ground state and low-lying states are described by strong two-state mixing, between normal and intruder configurations, involving distinct Elliott SU(3) structure and radically different deformation.