  The relativistic proton-neutron quasiparticle random phase approximation
(pn-RQRPA) is applied in the calculation of the L=0 strength in charge-exchange
reactions on $^{48}$Ca, $^{90}$Zr, $^{208}$Pb and nuclei in the Sn isotopic
chain. The microscopic theoretical framework is based on the relativistic
Hartree-Bogoliubov (RHB) model for the nuclear ground state. The calculation is
fully self-consistent, i.e. the same interaction is used both in the RHB
equations that determine the quasiparticle basis, and in the matrix equations
of the pn-RQRPA. The inclusion of the higher-order terms that include the
effect of finite momentum transfer, primarily the isovector spin monopole
(IVSM) term, in the transition operator shifts a portion of the strength to the
high-energy region above the Gamow-Teller (GT) resonance. The total strength is
slightly enhanced in nuclei with small neutron-to-proton ratio but remains
unchanged with increasing neutron excess. Based on the strength obtained using
the full L=0 transition operator in the pn-RQRPA calculation, we have estimated
the impact of the IVSM on the strength measured in the charge-exchagne
reactions on $^{90}$Zr and found that the data are consistent with the Ikeda
sum rule.
