  We present a tunneling study on single crystalline parent cuprate thin films,
i.e. a series of Pr2CuO4+/-{\delta}(PCO) with tunable superconducting
transition temperature. The zero-bias anomaly of differential conductance, well
reported in the normal state of R2-xCexCuO4 (R = Pr, Nd, La) and named as
normal state gap (NSG), is observed in the Ce-free samples. This NSG behaves
quite robust against the magnetic field up to 16 T, but fades away with
increasing the temperature. Most importantly, we find that the magnitude of the
NSG becomes larger with increasing point-contact junction resistance on the
superconducting films, which is further enhanced in the non-superconducting
samples of more oxygen disorders. The origination of NSG can be understood in
the framework of Altshuler-Aronov-Lee (AAL) theory, where the disorder-induced
electron-electron interactions suppress the density of states and thereby
result in a soft Coulomb gap.
