Parity mixing of pair at nuclear surface due to spin-orbit potential in $^{18}$F

We investigate the structure of $^{18}$F with the microscopic wave function based on the three-body $^{16}$O+$p$+$n$ model. In the calculation of the generator coordinate method (GCM) of the three-body model, $T=0$ energy spectra of $J^\pi=1^+$, $3^+$, and $5^+$ states and $T=1$ spectra of $J^\pi=0^+$, $2^+$ states in $^{18}$F are described reasonably. Based on the dinucleon picture, the effect of the spin-orbit force on the $T=0$ and $T=1$ $pn$ pairs around the $^{16}$O core is discussed. The $T=1$ pair in the $J^\pi=0^+$ state gains the spin-orbit potential energy involving the odd-parity mixing in the pair. The spin-orbit potential energy gain with the parity mixing is not so efficient for the $T=0$ pair in the $J^\pi=1^+$ state. The parity mixing in the pair is regarded as the internal symmetry breaking of the pair in the spin-orbit potential at the nuclear surface.