Effect of Energy-Dependent Proton Irradiation in Thin-Film YBa₂Cu₃O_7-δ Superconductor

The superconducting properties of YBa₂Cu₃O7-δ thin films were investigated by conducting 1.7 MeV proton irradiations with a total fluence of 2.64×1017p/cm2. The superconducting critical temperature (Tc) was reduced from 89.4 K to 10.1 K. The experimental procedure was similar to a previous study (0.6 MeV proton irradiation). We compared the effectiveness of Tc suppression by varying the proton energy from 0.6 to 1.7 MeV and found that in general both protons of 1.7 MeV and 0.6 MeV were effective in suppressing the Tc of YBCO. In particular, both results were consistent with the theoretical expectation (generalized d-wave AG theory) when a zero-temperature London penetration depth (λ0) = 215 nm is assumed for thin-film YBCO. For heavily irradiated cases (more than 80% Tc suppression), however, 1.7 MeV protons were more effective in suppressing Tc than 0.6 MeV protons. This can be understood by the fact that in the thin-film limit, higher-energy protons tend to produce less clustered point defects while lower-energy protons tend to create agglomeration of point defects.