Soft giant dipole modes in Ca isotopes in the relativistic RPA

The isovector giant dipole resonance in Ca isotopes is investigated in the framework of the fully consistent relativistic random phase approximation. The calculations are performed in an effective Lagrangian with a parameter set NL3, which was proposed for satisfactorily describing nuclear ground st ate properties. It is found that a soft isovector dipole mode for Ca isotop es near drip lines exists at energy around 6 similar to 7 MeV. The soft dip ole states are mainly due to the excitation of the weakly bound and pure ne utron (proton) states near Fermi surface as well as the correlation of isos calar and isovector operators. For nuclei with the extreme value of N/Z, th e contributions of isoscalar mesons in the isovector mode play a non-neglig ible role.