Low-energy Collective Gamow-Teller States and Isoscalar Pairing Interaction

The Gamow-Teller (GT) strength distributions and isobaric analog resonance (IAR) states of several N=Z+2 nuclei with mass number A=42–58 are studied by using a self-consistent Skyrme Hartree-Fock-Bogoliubov method plus quasiparticle random phase approximation (HFB+QRPA) formalism. The isoscalar spin-triplet pairing interaction is included in QRPA on top of the isovector spin-singlet one in the HFB method. It is found that the isoscalar pairing correlations mix largely the (νj>→πj<) configurations into the low-energy states, and this mixing plays an important role in the formation and in the collectivity of these low-energy states. Furthermore, the observed excitation energy of the low-energy GT state with respect to the IAR can be well reproduced when the strength of isoscalar pairing is about 1.0–1.05 times that of the isovector pairing, irrespective of the adopted Skyrme interactions. In N=Z+2 nuclei in the middle of the pf-shell, a mutual cooperative effect of isoscalar pairing and tensor interaction is found; namely, the tensor force reduces the spin-orbit splittings and enhances the effect of the isoscalar pairing.