  Multinucleon transfer processes in heavy-ion reactions at energies slightly
above the Coulomb barrier are investigated in a fully microscopic framework of
the time-dependent Hartree-Fock (TDHF) theory. Transfer probabilities are
calculated from the TDHF wave function after collision using the projection
operator method which has recently been proposed by Simenel (C. Simenel, Phys.
Rev. Lett. 105, 192701 (2010)). We show results of the TDHF calculations for
transfer cross sections of the reactions of $^{40,48}$Ca+$^{124}$Sn at
$E_{lab}=$ 170, 174 MeV, $^{40}$Ca+$^{208}$Pb at $E_{lab}=$ 235, 249 MeV, and
$^{58}$Ni+$^{208}$Pb at $E_{lab}=$ 328.4 MeV, for which measurements are
available. We find the transfer processes show different behaviors depending on
the $N/Z$ ratios of the projectile and the target, and the product of the
charge numbers, $Z_P Z_T$. When the projectile and the target have different
$N/Z$ ratios, fast transfer processes of a few nucleons towards the charge
equilibrium of the initial system occur in reactions at large impact
parameters. As the impact parameter decreases, a neck formation is responsible
for the transfer. A number of nucleons are transferred by the neck breaking
when two nuclei dissociate, leading to transfers of protons and neutrons in the
same direction. Comparing cross sections by theory and measurements, we find
the TDHF theory describes the transfer cross sections of a few nucleons
reasonably. As the number of transferred nucleons increases, the agreement
becomes less accurate. The TDHF calculation overestimates transfer cross
sections accompanying a large number of neutrons when more than one proton are
transferred. Comparing our results with those by other theories, we find the
TDHF calculations give qualitatively similar results to those of direct
reaction models such as GRAZING and Complex WKB.
