Unified mechanism behind the even-parity ground state and neutron halo of $^{11}$Be

Abstract

Using the axially deformed relativistic Hartree-Fock-Bogoliubov (D-RHFB) model, we explore the mechanism behind the parity inversion and halo occurrence in $^{11}$Be, which are well reproduced by the RHF Lagrangian PKA1. It is illustrated that evidently enhanced deformation effects by the $\pi$-pseudo-vector and $\rho$-tensor couplings in PKA1 are crucial for correctly describing both even-parity ground state (GS) and neutron halo of $^{11}$Be. Coupling with the deformation, the intrude $1d_{5/2}$ component largely enhances the couplings between the even-parity orbit $1/2_2^+$ and the nuclear core to promise the even-parity GS, whereas the $2s_{1/2}$ component therein dominates the halo formation in $^{11}$Be. Moreover, the deformed halo in $^{11}$Be is found to be stabilized by the attractive inherent correlations between the $1d_{5/2}$ and $2s_{1/2}$ components of the halo orbit $1/2_2^+$, instead of pairing correlations, which paves a new way to understand the halo pictures in deformed unstable nuclei.