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

The superconducting properties of YBa₂Cu₃${\mathrm{O}}_{7-\delta }$ thin films were investigated by conducting 1.7 MeV proton irradiations with a total fluence of $2.64\times {10}^{17}$${\mathrm{p}/\mathrm{cm}}^2$. The superconducting critical temperature ($T_c$) 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 $T_c$ 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 $T_c$ of YBCO. In particular, both results were consistent with the theoretical expectation (generalized d-wave AG theory) when a zero-temperature London penetration depth (${\lambda }_0$) = 215 nm is assumed for thin-film YBCO. For heavily irradiated cases (more than 80% $T_c$ suppression), however, 1.7 MeV protons were more effective in suppressing $T_c$ 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.