  Neutron-skins in coordinate space and proton-skins in momentum space are
predicted to coexist in heavy nuclei and their correlation is governed by
Liouville's theorem and Heisenberg's uncertainty principle. An analysis of
their correlation within a further extended Thomas-Fermi (ETF$^+$)
approximation incorporating effects of nucleon short-range correlations reveals
generally that protons move faster than neutrons in neutron-skins of heavy
nuclei.
