  Enhanced proton-neutron interactions occur in heavy nuclei along a trajectory
of approximately equal numbers of valence protons and neutrons. This is also
closely aligned with the trajectory of the saturation of quadrupole
deformation. The origin of these enhanced p-n interactions is discussed in
terms of spatial overlaps of proton and neutron wave functions that are
orbit-dependent. It is suggested for the first time that nuclear collectivity
is driven by synchronized filling of protons and neutrons with orbitals having
parallel spins, identical orbital and total angular momenta projections,
belonging to adjacent major shells and differing by one quantum of excitation
along the z-axis. These results may lead to a new approach to symmetry-based
theoretical calculations for heavy nuclei.
