  The enhancement of the sub-barrier fusion cross sections was explained as the
lowering of the dynamical fusion barriers within the framework of the improved
isospin-dependent quantum molecular dynamics (ImIQMD) model. The numbers of
nucleon transfer in the neck region are appreciably dependent on the incident
energies, but strongly on the reaction systems. A comparison of the neck
dynamics is performed for the symmetric reactions $^{58}$Ni+$^{58}$Ni and
$^{64}$Ni+$^{64}$Ni at energies in the vicinity of the Coulomb barrier. An
increase of the ratios of neutron to proton in the neck region at initial
collision stage is observed and obvious for neutron-rich systems, which can
reduce the interaction potential of two colliding nuclei. The distribution of
the dynamical fusion barriers and the fusion excitation functions are
calculated and compared them with the available experimental data.
