Point defect creation by low fluence swift heavy ion irradiation-induced low energy electrons in YBa₂Cu₃O_7-y

The effect of 200 MeV Ag ion irradiation on the superconducting and normal state properties of the high-T_c superconductor YBa₂Cu ₃O_7-y (YBCO) is studied by in situ temperature-dependent resistance measurements. We show that irradiating YBCO thin films (∼150 nm) at low temperature result in a softly defected region of about 85 nm radius due to swift heavy ion-induced secondary electrons around the highly amorphized latent tracks of ∼5 nm radius. This leads to a decrease of T_c at fluences three orders of magnitude less than the threshold fluence, where overlapping of tracks block the supercurrent path. Due to their low energy (4.1 keV for 200 MeV Ag ions), the secondary electrons can induce point defects by inelastic processes rather than by direct elastic collision.