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.