  For more than half a century, nucleons are considered to move continuously in
the nuclei. Recent electron-scattering experiments indicate about 20$\sim$25\%
nucleons in heavier nuclei are involved in the neutron-proton short-range
correlations. Nucleons in the nuclei thus have abnormal behavior unlike those
in the non-interaction fermi gas. In the neutron-rich nuclei, around the Fermi
momentum, the neutron-proton short-range correlations lead to a momentum gap in
the proton momentum distribution, which is circumstantially supported by the
pionic experimental data. Likewise, there should also be a neutron momentum gap
in the proton-rich nuclei. Nucleon momentum gap is thought to have profound and
extensive implications in the studies ranging from particle physics to neutron
stars as well as ultra-cold atomic gases.
