  Interplay between superconductivity and charge-density wave (CDW) in 2H-NbSe2
single crystals irradiated by 3 MeV protons is studied. Both Tc and TCDW are
found to decrease monotonically with the increase in irradiation dose. This
behavior is different from electron-irradiated NbSe2, where TCDW is suppressed
monotonically with the increase in dose, while Tc shows an initial enhancement
before it starts to decrease. We attempt to explain this difference based on
the negative pressure effect which has been reported in our previous study on
NbSe2 irradiated by heavy ions.
